Oh, it's wonderful. Emphatically, yes. "Good. I think it's one of my best ten dishes," says Mr Koyama with pleasure. "It is fugu."
Fugu. Not one of those plump, self-important little blowfish we patted in the Tokushima fish market at dawn? Fugu, £70 a kilo, four to each polystyrene carton where they sat in an inch of water puffing and pouting. Fugu with their white starched chests and elegant designer jackets which have this black blob centred in a white circle, a signal no doubt to predators: Eat Me At Your Peril.
So I have been eating this notorious, highly prized fish which every year in Japan kills dozens of unwary diners. They will, alas, have eaten fugu prepared by cooks not rigorously trained and licensed to do so. Paralysis sets in quickly if you should taste as much as a drop of its tingling poison. Although this is found mostly in the liver, one false cut will be enough to infect the rest of the fish. No wonder eating fugu has been likened to playing Russian roulette.
However, Hirohisa Koyama is licensed. He is also a chef able to elevate the fugu's delicacy to wondrous heights. He has expertly removed the poisonous liver, and it is the custard-soft roe of the male fugu he has used for this dish. So that's OK, then. And anyway, it's wonderful.
Did Mr Koyama himself eat a lot of fugu? Something may have been lost in translation (a mixture of mime, English, and offerings from his French- speaking sous-chefs). No, he didn't eat it often, he said: "Driving a car is quite dangerous enough." This sounded like a spin on the old Japanese proverb: "I would like to eat fugu, but life is too sweet."
The Japanese have been eating fugu for hundreds of years, though Koyama notes that only in the last 50 have they started eating it raw, along with other sashimi (sliced raw fish).
Putting your life at risk eating deadly fish, and eating raw fish, are but two aspects of a cuisine which is completely alien to most other food cultures. For historical reasons, Japanese cooking developed in isolation from the rest of the planet's, and in many ways is closer to the world of sixth-century Chinese Buddhism than to our own times. The Japanese do not breakfast on milky sugared cornflakes, but on savoury soup, rice and pickled vegetables. Westerners find their tastes strange because they relish bland white bean curd (tofu) as much as iodine-bitter seaweeds. Chewy dried squid is eaten from bags like crisps as a snack; a much-loved dessert is red beans cooked with sugar.
Yet Japan continues to exert a hypnotic fascination on many chefs in the West. The sculptural skills of its chefs has always amazed us, carving ice figures or turning vegetables into flowers. In France, in the 1970s, the decorative style of French Nouvelle Cuisine owed much to Japan - Paul Bocuse, one of the founders, was a regular traveller, bringing back revolutionary ideas. Others followed him; some have opened restaurants in Tokyo.
Then the tide turned and the first Japanese came to France. Two years ago Hirohisa Koyama arrived in Paris and thrilled 80 top French chefs with a precise display of knifemanship which would make a samurai warrior go green. He returned last year to wow them with demonstrations at the Ritz Cookery School.
And now Koyama is about to make his first culinary pilgrimage to Britain. He has been invited by Raymond Blanc, patron and chef of Le Manoir aux Quat' Saisons near Oxford, to cook during the first week in May to celebrate the opening of his Japanese garden, destined to be the third largest in Britain.
Hirohisa Koyama is 46, the first of a new breed of Japanese chefs to make a break with tradition. He may be modest, but he is not self-effacing, and he has built up a prodigious name for himself in Japan. In his book Aji no Kaze ("Windborne Flavours") he explains his innovative philosophy - to use the best ingredients, making them taste essentially of themselves, yet continuing to work within the traditional Japanese idiom. He rejects the universal Japanese practice of over-flavouring every dish with soy sauce, ginger and spring onion. This is the philosophy of the cookery school he opened three years ago, Heisei, where he teaches French and modern Japanese cooking.
Koyama's own renowned restaurant, Aoyagi, is not, as you might expect, in fashionable Tokyo or even Osaka, the business capital, but in the modest provincial fishing town of Tokushima. Initially, though, I met him in Tokyo. He is a beaming giant, standing over 6ft high, weighing some 16 stone. Sumo wrestling may not be his mtier but karate is. He is at once concerned to know how things are going in England. On the restaurant scene? No, no, the karate world. He strikes a menacing pose with raised fists. He knows (I don't) that the world karate champion in 1988 was an Englishman.
Hirohisa Koyama is staying at the prestigious Hotel Okuro, to talk with its venerable chef, Masakichi Ono. Mr Ono takes us on a tour of his French kitchen, past simmering cauldrons of stock (made with frozen blocks of Australian beef) and then into the Japanese kitchen. There is a moment of wonder as Mr Koyama watches with professional approval a young chef who grips a huge Japanese white radish (daikon or mouli) in one hand and turns it against a heavy silvered blade. A 6in-wide ribbon of radish unravels. Already it must be at least a metre long. How long can it get? "If the radish is big enough, six metres," says Koyama.
The next day, we flew from Tokyo's squeaky-clean, new domestic airport, lunching on an international hybrid, a very un-Japanese dish enigmatically named "cutlet"; a breaded escalope of pork, deep-fried, with rice and curry sauce. It was surely not a comment on the food that we walked out without paying and had to be called back. Serious loss of face.
Tokushima is the third largest city (population 850,000) of Shikoku, the fourth largest island of Japan, a fishing port straddled by a dozen parallel rivers flowing into the straits between the islands, spanned by Japan's longest bridge.
Koyama's restaurant, Aoyagi, in Tokushima's street of restaurants, belonged to his parents and grandparents. It is a low, traditional wooden structure, modestly tucked behind a weeping willow and bamboos, and as quiet as a temple. It was here, in his adjoining Basara Bar where prices start at around £100 a head, that I first ate fugu.
The previous night I'd been treated to an extraordinary meal in Aoyagi (dinner here starts at about £200 a head), where Hirohisa Koyama practises what many believe to be the finest modern Japanese cooking, in the style they know as Kaiseki - what we would call haute cuisine.
In the low-roofed building in a room decorated simply with a poetry scroll, a flower arrangement and a smiling carved Buddha, we sat on bamboo tatami mats at a low table, a kimonoed attendant serving each person. We tucked in to what seemed to be a 50-course meal, an improbable experience, since no flavour appeared to be repeated more than once.
It may not have seemed so, but this was actually a formal Japanese meal. There was a fixed number of courses, nine, but within each one there were numerous miniature dishes - a dozen in the first course alone, the appetisers.
The picture on the right shows the style of presentation. It can't convey the flavours and textures, though they followed the rules of any good cooking - a delicate balance of sweet and sour, of salt and bitter, pungent, crunchy, slippery.
The ingredients were something else: fish, of course; some meat (even foie gras); some predictable vegetables such as bamboo shoots, and shiitake mushrooms; and some unfamiliar herbs and seasonings, such as seven-spice powder, shichimi togarashi (ground black pepper, dry chillies, poppy seeds, sesame seeds, hemp seeds, dried orange peel and nori seaweed powder). There were other stranger ingredients it might have been better not to ask about; but such is Koyama's artistry you do not go pale when you find you've been eating mashed 1,000-year-old egg (Chinese fermented eggs) or slimy green strands of sea-slug stomach.
Quite wrongly, I thought I had known roughly what to expect. I had actually been boning up for this occasion in Tokyo, guided by an expert on the city's restaurants, noted gourmet and Pommery champagne importer Ryujiro Ishikawa. He had explained that Japanese restaurant cooking is sub-divided into some 30 categories, some styles well known in the West, like teppanaki, where the chef struts his stuff at a stainless steel top in front of diners. There are sushi bars (where you buy not inexpensive pieces of raw fish pressed into rice), sashimi bars (raw fish slices) and eel bars selling grilled eel. There are yakitori bars, where skewered meat and fish is grilled over charcoal, and oodles of soup and noodle bars, divided into udon (fat white noodles) and soba (buckwheat noodles).
And that's not to mention hot-pot. There are sukiyaki bars which offer one-pot meals with meat, and shabu-shabu bars where you dip razor-thin slices of beef into the hot-pot, and oden bars with vegetarian tofu and seaweed hot-pots. And so on. There are tempura bars (deep-fried vegetables and seafood in batter) and bars which do fry-ups, bento shops selling hot takeaway lunch-boxes, and many bars offering a dozen regional styles of cooking. And so on.
Kaiseki embraces most of these styles and then some. But in miniature. What you might call bonsai cooking. At Gora Kadan, a luxury hotel with hot volcanic springs tucked away in the foothills of Mount Fuji, Ryujiro Ishikawa talked me through the nine courses of a classic Kaiseki meal. A tray of six or more appetisers will be followed in rigid order by: Soup. Sashimi (raw fish). Grilled fish. A simmered dish. A deep-fried dish. A steamed dish. A salad. Then the trinity that concludes all Japanese meals - soup, rice and pickles. Lastly, dessert. The meal concludes with a bowl of astringent, bitter green tea, made from the unfermented leaf, powdered. There is nothing like it to snap those taste buds sharply shut.
Each dish is served on a different plate. Every saucer and bowl contrasts in design, shape, colour and texture, from ceramic, porcelain and glass to china and lacquered-pine. Many are decorated with seasonal flowers (peach and pear blossom for March, cherry for April and so on) and consequently all the crockery has to be changed four times a year to match the seasons. Gora Kadan's managing director, Yuji Fujimoto, sighed and confessed that they needed two large rooms the size of the dining room itself to accommodate this vast store of crockery. At this level, it is what his paying customers expect (but pay they do).
So it came as no surprise, dining at Hirohisa Koyama's Aoyagi, to be told that the pretty blue plate I'm admiring (on which he has served a piece of grilled sea bream worth a few pounds) is an antique from his mother's collection valued at £4,000. It was painted in the 19th century by a descendant of Sen-no-Rikyu, the founder of the noble Japanese tea ceremony from which stem all notions of social order at the table.
Koyama didn't at first plan to follow his parents into a business which was essentially country cooking. He headed for the bright lights of Tokyo to study engineering at technical college. He changed his mind, but decided he needed professional training. He was taken on by one of Japan's most distinguished chefs, Teiichi-Yuki (who is now 94).
What sets the professional chef apart from the ordinary in Japan is skill with the knife. The art of using the larger, dangerous, heavy carbon steel kitchen knives (hocho) is essential to classical Japanese cooking. As Koyama puts it in his book: "First there is cutting. Cooking follows."
He says it took him years to achieve a real understanding of cutting sashimi. "No matter how sharp the knife is, the final state of the cut surface, seen on a microscopic scale, is jagged like a saw edge. The difference is the degree of jaggedness - that is, rough or finely jagged. If the cut is made with a rough edge, the fibres of the fish will be squashed and the essence will run out. A clean cut leaves the surface satin-smooth." He puts a fillet on his cutting board, and bears down, slowly and evenly, on his hocho blade. He stops: "Look at its perfect muscle tone." The fish quivers. He draws the blade away, leaving the exposed surface shimmering.
One of his chefs illustrates the technique for cutting raw squid to make it digestible. He takes a sheet of smooth white squid, trimmed to a rectangle. With his 12 in-long blade he makes some 150 diagonal, shallow cuts on each side. He cuts each piece into half the size of a matchbox, and pinches them into shapes, piling them with other pieces of finely cut fish, salmon, tuna, sea bream.
Ah, sea bream. Koyama believes the purest ingredients must stand on their own. And no sea bream has firmer flesh or better muscle tone than the local reddish-gold Naruto fish which ply the eddying currents offshore. Koyama's Paris-trained deputy chef, Hiroyuki Kanda, drives me to the centre of Japan's longest bridge, linking Shikoku island to the mainland. We hang on to the silvery rails, braced against gusting winds, and look down at the boiling ocean where the Oxford blue of the Inland Sea of Japan crashes into the Cambridge blue of the Pacific.
This is the point where the local fishermen catch the finest Naruto sea bream. They sail into this cauldron and hurl overboard their traps baited with sand eels. It takes fine steering skills to measure the speed of the current to calculate the time it will take the trap to hit the bottom.
If they miss, they sail on, and the next boat follows. Koyama describes the process in his book: "The fishermen take turns, going in order as if in a Yabusame rite, samurai warriors shooting arrows at marks off the backs of galloping horses. Only because of the perception and steering skills of these fishermen can these fine fish be delivered into the waiting hands of the chefs. It is our duty, therefore, to prepare the Naruto sea bream with well-sharpened knives." Hirohisa Koyama, cook, engineer, karate champ, poet.
! Chef Koyama will be cooking lunch and dinner from 1-5 May at Le Manoir aux Quat' Saisons, Great Milton, Oxford, as part of a Japanese arts festival. Telephone 0844 278847 for details.