If you think you know all about Thai food, eating at Nahm will make you think again. Sybil Kapoor meets David Thompson, who has astounded diners with the results of his quest for authenticity

We think we've got Thai food taped. After all, green Thai chicken curry has become as much a part of the national diet as bacon and eggs. It's everywhere – on pub menus, in the supermarket chill cabinet, probably even in railway buffet sandwiches. It actually outsells Indian curries in some quarters.

The familiar cocktail of lemongrass, curry and lime leaves, coriander and coconut milk that is a common denominator of some basic Thai food may appeal to our palates, but how much do we really know about the cuisine? Having tried the latest Thai restaurant in London, I'd say it seems we know next to nothing.

When Stefano Cavallini left his eponymous restaurant at The Halkin hotel in Belgravia, it needed to replace him with another high profile chef. Perhaps it realised we're developing a taste for Thai and are ready to extend to our knowledge; perhaps it was lucky that the English-speaking world's leading authority on the South-east Asian kingdom's cooking was ready to leave Australia. David Thompson is revered for his knowledge of Thai food. His Darley Street Thai restaurant in Sydney had, over a decade, changed attitudes and won accolades, but he had sold it and was looking to move somewhere he could continue to introduce diners to a repertoire he's been developing for 15 years. The Halkin invited him to come to London.

Less than a month after he opened Nahm, Thompson's uniquely complex, classical Thai cooking, unlike anything we've experienced before, was astonishing some and baffling other diners. Over-priced and over-sweet, muttered its critics. Whoever heard of paying £16.50 for a main course in a Thai restaurant? Or, worse still, eating a sweet starter such as his subtly flavoured, sugary ground salmon, accompanied by juicy morsels of watermelon on betel leaves?

Australia may have discovered Thai food before we did, but even there it took time for Thompson's efforts to be understood. He himself admits that he took a while to come round to Thai. "I started mucking around with food when I was a child," he says, "but my mother was an abominable cook. She made gravy with wholemeal flour, for God's sake – no wonder it was lumpy!"

His obsession for cooking finally took root after university. "My first job was in 1982, at a French restaurant called La Goule, in Sydney," the 41-year-old chef continues. "As far as I was concerned, French food was über alles, and I was prepared to suffer under real bastards to learn how to cook it."

Thai food did not impinge on his culinary world. "I used occasionally to eat some awful, rubbery Thai fish cakes from a local café, and I liked coriander, although I thought lemon grass was really overblown and coarse, best used for swatting flies," he says, showing as much flair for talking as for cooking. Then, in 1986, he went on holiday to Thailand. He decided that he wanted to live there.

Back in Australia, he bought Jennifer Brennan's seminal, and still not bettered, Thai Cooking. Still he felt that Thai food couldn't compare to the sophistication of Fredy Girardet or the Troisgros brothers. Six months later, he returned to Bangkok, eager to open a European restaurant.

Nothing came of this ambition, but during the two years he spent there he finally succumbed to the cooking, as well as the country. "I was gobsmacked by a meal I had at a friend's house," he recalls. "I was given an appetiser of minced prawns and chicken simmered in palm sugar with deep-fried shallots, garlic and peanuts, on little pieces of pineapple and mandarins. The clarity of the flavours was amazing." The cook was an old woman called Khun Sombat Janphetchara, who had learnt to cook at court. Recipes are rarely written down in Thailand, but classical cooking traditions are carried on in the royal household. Thompson struck a deal with her: he paid for all ingredients and for six intensive months cooked under her supervision. He also began to collect Thai memorial books, which are published when a person dies and include a wide selection of the deceased's favourite dishes. They offer one of the few written sources of classic recipes (albeit without any quantities given). The archive that Thompson has collected ever since includes books dating back to the 19th century.

Armed with this remarkable training and his collection of recipes, he returned, when his money ran out, to Sydney, where his Darley Street Thai restaurant opened up a whole new dimension of Thai cooking to Australians.

There are many aspects of Thai food that the English-speaking world still does not understand, he believes. Rice, for instance, is the key element of a Thai meal, and many of the intensely flavoured, fiery accompaniments that are commonly eaten here as a main dish should really be eaten as a relish to add savour to the rice. "Thai cooking is based on three things: the actual taste of the ingredients, their texture and the seasoning – in other words sweet, sour, salt and bitter," Thompson explains.

"The seasoning is used to bring out the flavour of the ingredient, but is far more complex than in the West. They have five or six different types of sugar, for example, such as coconut palm and palmyra palm sugar – all of which vary in taste and texture from month to month."

In London, and in an ideal world, he would serve only a choice of set meals (£47) – so that customers were presented with their food in the correct way. Thus soup, rice, curry, stir-fry, grilled dish and salad appear at the same time. Each is designed to counterbalance the other, so that the diner can refresh themselves with a clear samphire soup, for example, before tucking into sour grilled-aubergine salad with eggs and dried prawns. The key is not to clutter the palate by filling the plate with lots of different foods at the same time.

"It is amazing how unsophisticated our palates are," the chef says. "Many people actually get quite angry if they don't like something. It's almost as if anything unfamiliar is poisonous." Given that Thai cooks use sugar in much the same way as we might use salt, some customers have been thrown by his menus. Others, however, rave at the succession of new tastes. Jeremy Lee, head chef of the Blue Print Café, is one of the new restaurant's fans: "He puts all his effort into the cooking and creating fantastic flavours, rather than serving the usual foie gras with spotty-sauced muck. It took 50 years for a superior form of Indian cooking to become accepted here, so it may take some time for people to accept Nahm."

Thompson has brought his second-in-command Australian, Damien Jones, and his Thai pastry chef, Tanongsak Yordwai, with him to London. Unusually among globe-trotting chefs, he is based here, rather than flitting back and forth as a consultant, and cooks the repertoire of which he has made himself a custodian. Next year his cookery book will be the definitive collection of traditional Thai dishes. He plans to create a library of historical recipes from the memorial books, and eventually make his home in Thailand. In the meantime, London is lucky enough to have what must be one of the best Thai restaurants in the world.

Nahm, the Halkin Hotel, 5 Halkin Street, London SW1 (020-7333 1234)