The French Laundry, in Yountville, California, has been described as the best restaurant in the world. Hyperbole? That's what Andrew Gumbel thought - until he dined there. Now, he says, nothing will ever taste the same again

T here is something strangely daunting about going to the best restaurant in the world. Daunting, and a little unnerving. After all, as your stomach growls from the day-long fast you've imposed on it, and you check your watch for the umpteenth time to count down the minutes until your reservation time; and your mind focuses with vague anxiety on the very large chunk of change with which you are about to part; and you begin to question the wisdom of travelling half-way up California into the heart of the Napa Valley just for a meal, for crying out loud... it's hard not to wonder: just how good can the French Laundry be?

Can it be better than that magical lunch near Reggio Calabria, with skewered involtini of swordfish straight out of the Straits of Messina, and a sorbet of locally grown lemons so intense and sweetly pleasurable that it made me wonder if I'd ever truly eaten a lemon before? Better than the meze feasts right out of The Arabian Nights that I experienced across the Levant? Better than the startling simplicity and zest of La Super-Rica, Santa Barbara's disarmingly modest taco-stand extraordinaire?

I'll be honest and state straight away that my taste in food tends toward the peasant end of the spectrum: dishes bearing the imprint of their local origin, made with prime fresh ingredients and cooked effectively but without fuss. I've always regarded the temples of haute cuisine as a bit of a rip-off: too much pomposity and fussy presentation for too little pay-off where it counts, on the plate.

Then again, I always fancied I knew quite a lot about food. The most sobering thing about eating at the French Laundry - and I'm far from the only one to undergo this epiphany - is the realisation that, in fact, I know next to nothing. Whether Thomas Keller's Californian country establishment deserves its title - recently conferred on it by Restaurant magazine - as the best in the world is perhaps beside the point; Keller certainly never asked for the label and, to the extent that it's a distraction from the food, probably finds it just as unnerving as I did. What the French Laundry does, however, is to redefine entirely the concept of eating-out. The food is not just better made or more innovative or more beautiful than anyone else's; it actually makes you reconsider your entire relationship with what you pick up on the end of your fork and place in your mouth. Long after digesting the last morsel of my extraordinary meal, I'm still chewing it over and trying to understand what happened there.

The French Laundry, on the edge of the tiny town of Yountville, looks, at first glance, little more than an unassuming residence. (It was once a steam laundry, hence the name, as well as a saloon and a brothel.) There is a sign, but it is sunk into a stone wall and concealed by climbing plants. We sauntered up at 5.30pm (a preposterous time to have dinner, I thought, but that was before I knew what was coming) and could have sworn the place was deserted. The only entrance that was open was the gate, leading us to a magnificent but empty back terrace and garden, dominated by a Chinese hackberry tree. Had we come on the wrong day? Was the restaurant even open?

Gingerly, we tried a side-door and came into a small bar area looking on to the downstairs dining-room. Everything was set for dinner, but, as on a ghost ship, there was no sign of human life at all. At length, a hostess appeared and explained that it would be a few minutes before they would be ready to seat us. Other diners began to float into the garden and seemed equally lost. In retrospect, the effect was deliberate: we were made to feel that we were entering another world.

Soon, waiters in black ties and long aprons were offering us champagne and an appetiser that looked for all the world like an ice-cream cone, a cornet filled with salmon tartare and sweet red-onion crème fraîche. Again, the effect was deliberate: to disarm us, loosen us up a little, even amuse us. As I later learnt, Keller got the idea from a visit to the down-market ice-cream chain Baskin-Robbins. Who wouldn't want to start a meal with a childhood memory of munching on an ice-cream cone?

Menus arrived, offering a staggering number of dishes spanning either five or nine courses. I thought the choice of what we ordered might matter, but in the end it scarcely did. Keller, I later realised, was interested in giving us the notion of choice, but really his intention was to blow us away, on his terms, at his pace, with the dishes that he felt like serving. What did it matter, in the end, if we ordered the wild bass with forest mushrooms or the poached Maine lobster with pea shoots and carrot butter, when either of them would open us up to tastes and sensations we could never have guessed at simply by reading it off a printed list?

I opted for the five-course menu, while some of my fellow diners went for the full nine. In the end, we all ate at least 12 courses, possibly more; by the end of the five-hour marathon, I was too dazed and addled even to keep count. There was the extraordinary "sandwich" of soft-shell crab on top of a toasted brioche, with greens and a tomato confit, spiced with a French pickle sauce that looked uncannily like mayonnaise but tasted nothing like it at all. Or the beautiful "oysters and pearls" - an unorthodox combination of tapioca, oysters and caviar on a sabayon, or savoury egg custard, served with a custom-made spoon of mother-of-pearl. Or the sheer indulgence of foie gras with black truffles. And those were just the delights of the opening half-hour.

Each course gives you just enough to appreciate the intensity of the flavours, both individually and in combination, and then it is finished, right at the moment when you are utterly falling in love with it. That is all part of the Keller method. As he describes it himself in The French Laundry Cookbook (Workman, £35): "What I want is that initial shock, that jolt, that surprise to be the only thing you experience - I want you to say, 'God, I wish I had just one more bite of that.' And then the next plate comes, and the same thing happens, but it's a different experience, a whole new flavour and feel."

It quickly becomes apparent that the food is less about satisfying appetite than it is about spectacle and performance. Everything, from the cutlery to the way the waiters put the plates down simultaneously at two places, has been thought through and carried out according to a supremely assured director's instructions. As the evening went on, I thought of it in terms of a grandly ornate opera in which one swooningly beautiful aria is succeeded by another reaching even greater heights. I thought nothing could touch the incredible striped-bass-and-multiple-mushroom combination, but that was before, several courses later, I tasted the mango sorbet and thought to myself, "I will never eat anything except dessert ever again."

Eating at the French Laundry is not unlike listening to Wagner's Tristan und Isolde: hours and hours of mounting sensory stimulation in anticipation of a final tonic chord resolution that keeps promising to arrive but is endlessly deferred.

Unlike Wagner, though, Keller has a palpable sense of humour. He grew up without culinary pretention, and many of his dishes parody the fare of his Middle American youth. "Bacon and egg" is tête de cochon and a poached quail's egg, intriguingly served on a silver spoon rather than a plate. "Fish and chips" is red mullet with a palette d'ail doux and garlic chips, sitting atop a bright-green parsley coulis. He also does a "soup and sandwich"; the "peas and carrots" I tasted with the Maine lobster (unlike any combination of peas and carrots you've ever had); and even a macaroni cheese. For dessert there is "coffee and doughnuts" (a cappuccino semifreddo with cinnamon-dusted doughnut balls) and, as a digestive mignardise, "peanut butter and jelly", peanut-flavoured chocolate truffles and French-style jellied fruits.

More important than the playfulness, though, is the extraordinary attention to detail. Nothing is so trivial that it has not been thoroughly thought through, and nothing so marginal that it can be safely ignored. In stark contrast to most high-class kitchens, with their constant tantrums and their slop of sauces and vegetable ends spilt on the floor, Keller and his staff work in a state of complete calm, in a kitchen so pristine that the pots and pans are scrubbed at all times to look as though they are brand new. (They end up being thrown out because they no longer sit straight on the stove from all that scrubbing.) Keller himself is often sweeping, or cleaning the flavoured-oil bottles, or, if he saunters outside, picking up cigarette butts left in the driveway.

He believes in blanching vegetables in hard-boiling water so salty it should taste "like the Atlantic Ocean", the idea being that the faster you cook vegetables, the more they retain their colour and flavour. If he sees the water lose its boil, he will ask the offending sous-chef to throw out the vegetables and start again. Now that's attention to detail. This is a man who extracts chlorophyll from plants to make sauces, who doesn't let liquid travel from one container to another in his kitchen without first passing through a strainer, preferably several times over. It's all about purity, and precision. When he orders fish, he asks his purveyor to pack and transport the fish in the direction in which it was swimming when it was caught. One might easily laugh at this and ask why, but Keller's response would be to counter, why not? Why stress the flesh of the fish unnecessarily, if you don't have to?

It comes down to the old adage that, if something is worth doing, it's worth doing properly. Keller learnt this from his mother, who trained him to aim for perfection in his household chores, and he has applied it to everything since. In his first job, cooking steaks and breakfast at the Palm Beach Yacht Club in Florida (his mother was the manager), he became fascinated with making the daily hollandaise sauce for eggs Benedict. For two years, he focused on the hollandaise, seeking to wheedle out its mysteries and improve his technique each time.

From there, he served apprenticeships in France, Italy and upstate New York, before opening his first restaurant, Rakel, in New York City in 1986. He earned rave reviews, but his clients deserted him after the 1987 Wall Street crash. He bounced from job to job, landing in Los Angeles for a while before the seminal moment in 1992 when, maxed out on his credit cards and, essentially, unemployed, he first clapped eyes on the French Laundry and knew he had met his destiny.

It took 19 months to assemble the financial backing he needed, by which time he was so broke that he and his skeleton staff had only eight pots and no sauté pans at all. The business was touch and go until 1996, when the Laundry first started getting serious critical attention. (That was also the first year it turned a profit, of just $17.) Today, demand for tables is so great that would-be diners are told to call two months to the day before they want to make their reservation. Reservations are taken on a single phone line, which is almost constantly engaged.

Later this year, Keller will embark on a new adventure - opening a branch of the French Laundry in New York, in a custom-designed space on the fourth floor of the new AOL Time Warner Centre in Columbus Circle. The exact opening date depends on the construction schedule, probably November or December. To ease the transition, the Yountville branch will close for a few months for renovation.

Eventually, Keller will install two of his most talented protégés, Eric Ziebold and Jonathan Benno, in charge of day-to-day operations in Yountville and Manhattan respectively, while he flits between the two. With anyone else, one would have to wonder whether such towering standards could be maintained at two locations that are 3,000 miles apart. With Keller, though, it's hard to imagine the enterprise being carried off any less than perfectly. If he can juggle 50 or 60 dishes on a menu each night - double or even triple the burden most top-class restaurants impose on themselves - then going bicoastal can't be that much more of a stretch.

My own encounter with Keller's cooking left me so dazed that I wasn't sure at first what to think. Not everything about the experience was entirely pleasant, at least at the time. I had no complaints about the bill, which at $200 a head excluding wine seemed rather reasonable. But the food demanded such attention that normal conversation seemed almost impossible. We talked mostly about the restaurant itself, which required us to go silent like naughty schoolboys whenever the waiters loomed. When it was over and we staggered out into the deserted Yountville night, we were so full we could scarcely stay awake on the hour-long drive back to San Francisco.

I didn't eat a thing for 24 hours, and didn't feel genuinely hungry for another 48. My first impulse, when, finally, I felt like it, was to tear into an ordinary roast chicken and vegetables, and to relish the fact that there was no pressure to admire everything as I popped it into my mouth. But that sense of liberation didn't last. As one of my fellow diners remarked, several days later: "After you've eaten at the French Laundry, nothing else seems to taste right."

Keller has said that his restaurant can never attain perfection, but it can at least be close enough to put the idea of perfection within reach. I haven't stopped thinking about that, or about the deeper implications of his philosophy for the kitchen, and beyond. And the more I think, the more I want to go back to the French Laundry again.