Somerset House is a bastion of Englishness. So you'd expect The Admiralty to be as English as plum duff. Not so. It embraces, of all things, French cuisine

In the six or so weeks that Somerset House has been open to all for bread and cuisine de terroir, every other member of the restaurant reviewing rabble has been and reported back. I used to imagine I'd be first to storm the barricades, so shame on me for bringing up the rear with news of The Admiralty.

In the six or so weeks that Somerset House has been open to all for bread and cuisine de terroir, every other member of the restaurant reviewing rabble has been and reported back. I used to imagine I'd be first to storm the barricades, so shame on me for bringing up the rear with news of The Admiralty.

You probably know that 18th-century Somerset House - which served as naval offices at the height of Francophobia, then housed the register of births, deaths and marriages, and offices of the Inland Revenue - has been thrown open to the public in the millennial mood of art for all. The central courtyard has been liberated from its role as car park for tax collectors and awaits the removal of temporary barriers around what will be dancing fountains to become a spectacular public space. One wing now houses the Gilbert Collection of gold and silver, while on the southern side of the magnificent quadrangle is the restored Seamen's Waiting Hall, where naval officers awaited orders for battle against the French.

During the day this grand lobby hung with huge portraits of naval heroes, notably the French-hating Nelson, is a thoroughfare between the square and the river terrace where tables under tented awnings form an all-day café (and where smokers can go after dinner) as long as summer lasts. At night it echoes to the intermittent clack of customers' heels on the stone floor, as an awe-inspiring entrance to the splendid congress of past and present which is The Admiralty.

With an Australian architect, a French-Moroccan designer and a French chef, Irish restaurateur Oliver Peyton has created a restaurant of boundless quality in an institution that was once a bastion of Englishness. Even the recorded message has a European accent - and told me the restaurant was fully booked when I first wanted to go (hence the delay).

Even if you fail to book you can still drop anchor at the bar, the first of three high-ceilinged rooms respectfully painted in rich matt National Trust tones. One wall is furnished with stacks of wine bottles; from behind the bar comes a peerless gin fizz. The two adjoining dining rooms are decorated with a turtle's carapace, stuffed crocodile and ibis head, as if brought back from a territory-charting voyage. There are naval prints and, most enchantingly, glass chandeliers in the shape of sailing ships, which cast a dimmish light on modern turquoise chairs and unclothed dark-wood tables. The noise level is that of the social battleground.

Each table sports a little platform. To take advantage of this, order the mixed hors d'oeuvres or selection of rustic terrines served in little dishes, which are lined up on the raised ledge leaving the smallish tables less cluttered. But first, an irresistible warm and crisp torpedo-shaped wholegrain loaf takes its place there while you read the menu, written in French with English translations. It exposes the myth that modern British cooking can compete with the more established cuisines of the world. What a joy not to find fishcakes and Caesar salad on a menu.

With the hors d'oeuvres comes a bowl of preternaturally shiny, albeit organic, vegetables, as is the way in Lebanese restaurants. On top of peppers, tomatoes, carrots, raw asparagus spears and lemongrass stalks, sits a single egg. Are we supposed to eat or admire this? Do they hose it down for the next customer? No matter, for there are also (for £8.50 per person) little pots of pesto, tapenade, and dishes of piquant cauliflower, flageolet beans with bacon, artichoke hearts, lobster, baba ganoush and more.

Each was very fine, but the effect was a touch too like a wine bar. "Not what I want from a restaurant," captiously muttered my consort who likes the first of his three courses to be more conventional and less vegetal. Not one to leave food uneaten, he popped the egg, which he hoped was hard-boiled, into his pocket to take home.

After an interval slightly longer than decent came monkfish roasted on the bone to a slightly more cardigan-like than worsted degree of woolliness. This was compensated for by the array of miniature vegetables around it. Representing the variety and meticulousness of supply that only a restaurant dedicated to the best can conjure up, there were aubergines, asparagus spears, onions and courgettes so sweet and tiny they made the roasted cherry tomatoes look like cannonballs.

They call this cuisine de terroir - French for evolved earthy cooking - and each dish balances fish or meat beautifully with seasonal vegetables. They're served in quantities unstinting enough to keep scurvy at bay, as the light-fingered knave commented. His pigeon infused with fresh thyme and with sweet petits pois a la Française, which patently weren't provided by that latter-day seafarer, Captain Bird's Eye, had all the bosky fragrance and flavours of rural France.

While The Admiralty gently and stylishly flouts some of the conventions of traditional restauration (no tablecloths, staff who talk back), it idiosyncratically reintroduces a few. And, like the turquoise, leather- bound wine list, although it is not strictly necessary to have the waiter spoon a sweet stew of beautiful peeled broad beans from a copper pan on to a plate bearing a magnificent veal chop, occasional instances of silver service feel appropriately respectful both of the surroundings and the cooking.

The same treatment is given to a sensational chocolate mousse. It is borne to the table in a large stainless steel bowl, and there is a tantalising delay as one, two, even three billowing, dusky serving spoonfuls are transferred to the plate. No other adorning flourishes were needed, not when, within the heavenly coalescence of chocolate, eggs, cream and air are suspended bitter chocolate filings to create crisp resistence against the melting background. However, should the mousse appear bare without any accompaniment, a plate of shell-shaped pistachio madeleines was provided. More virtuous, though no less an indulgence, was a bowl of soft fruits in Monbazillac with apricot sorbet on top, which raised fruit salad to a dizzyingly delicious height. Coffee then came with dangerously good dark chocolates.

Altogether admirable, The Admiralty shows how far London has come now some of the barriers are down. Our dinner left us £50 each worse off, which even laggardly riff-raff like us thought not too high a price for such a gracious and delicious entente.

* The Admiralty, Somerset House, The Strand, London WC2 (020-7845 4646). Mon-Sat, lunch 12-2.45pm, dinner, 6-11.15pm; Sun lunch 12-2.45pm. All major credit cards. Disabled access