The fish that ate London

The signature dish is a 21st-century gastronomic must-have: the restaurant world's answer to the personalised numberplate. At its most ingenious, this dish encapsulates the quintessence of a restaurant. The most modern example of this is, of course, Nobu's Black Cod. For almost five years Black Cod has been the colossus of signature dishes.

To take just the first exquisite mouthful of the rare Alaskan fish, marinated to give it a unique, sweet flavour, is to understand at once the reason for its iconic status. The dish relies on an ingredient with which we are familiar, but renders it entirely fresh and new through imaginative treatment... much as Nobu did with the concept of the London restaurant. Rather like an edible definition, the best signature dishes tells you everything you need to know about a place.

Moreover, for the gastronome who likes to seem au fait with everything, even if actually knowing very little, the signature dish also makes life easy - you know what to order and what to brag about. In the earlier stages of its development, the signature dish was usually something outlandish, whistled up to mark a special event. These celebrity-endorsed dishes came to the fore in the 19th century. The Stroganovs, a wealthy Russian merchant dynasty, commanded their French chef to invent them something catchy, rich and red-blooded; hence Beef Stroganoff.

By the end of the 19th century, the signature dish had gained such popularity that superstar soprano Dame Nellie Melba had two named after her: Melba Toast and P?che Melba. The former was a piece of toast sliced horizontally to make a brace of thin wafers, the latter a pudding of vanilla ice cream, peach and raspberry sauce, created for her in London in 1894 by fin de si?cle superchef Auguste Escoffier after the chanteuse gave him a couple of tickets to see her in Lohengrin.

There still exists in London at least one classic example of this highly stylised, 19th century-style signature dish: Sole Jubilee, a fillet served in a lavish pastry case, created in 1977 to mark the Silver Jubilee of Queen Elizabeth II. It remains on the menu at The Connaught. What it will cook up for next year's Golden Jubilee remains to be seen.

The father of the modern signature dish was Arrigio Cipriani, proprietor of Harry's Bar in Venice, who set the tone for the latter half the 20th century with his Beef Carpaccio. In the autumn of 1950, the contessa Amalia Nani Mocenigo walked into his restaurant and announced that she was on a diet that prohibited cooked meat.

The ever-resourceful Cipriani had the kitchen whip up a plate of finely sliced raw filet mignon, laced with a white sauce of mayonnaise and mustard. 'And what is that?' asked the countess. 'A Beef Carpaccio,' replied the restaurateur, as if the dish had been around for years. In fact he had just made it up, giving it the name of the Renaissance painter who was the subject of a big exhibition in Venice that year, and whose works contained a lot of red and white.

Although the inspiration came to him in an instant, Beef Carpaccio celebrates its 51st birthday this year, having become a genuine timeless classic and a dish that has made Harry's Bar a place of gastronomic pilgrimage for the jet set. Michael Chow achieved a similar result in Knightsbridge in the early Seventies when he put green prawns on the menu of the restaurant that bears his name - Chow was even painted as a green prawn by New York artist Keith Haring.

It is, however, notoriously difficult to predict which dishes on a menu, if indeed any, will achieve immortality. But in London over the past two decades they have tended to be divided into two categories: the simple and the sophisticated.

The simple signature dish is predicated on the basis of a sort of inverse snobbery: typical of this sort of cult cooking is the lobster club sandwich which is served at Christopher Gilmour's eponymous Covent Garden restaurant.