Green Man and French Horn, 54 St Martin's Lane, London WC2

I didn't do too much research before setting off for lunch at Green Man and French Horn. All I knew – all I needed to know – before piling in to this Covent Garden newcomer at the earliest opportunity was that a) it's the latest opening from those clever young chaps behind Terroirs, and b) they are very good at creating fine places to eat.

I assumed their latest, a converted pub on St Martin's Lane, would follow in the tradition of Terroirs, their rebooted wine bar and the local heroes it spawned, Brawn and Soif, all built around a winning formula of natural wines and meaty, French-inspired food.

But there's something different about the Green Man. There, on the arty, hand-drawn menu, nestling among the rillettes and roasts, is seafood. And several varieties of fish. And vegetables. What the foie? Could it be that those butch boys have gone all girly…?

In fact, their latest venture is inspired by a specific terroir – the Loire. Ah yes, the Loire. (Rummages through bluffer's guide under the table.) What do we know about the Loire? They make wine around those parts. It's a river – possibly the biggest river in France? It kind of goes up the middle and round to the side [citation needed].

While most of us could probably muster up a few rudimentary facts about the cuisine of Provence, Bordeaux or Lyon, the Loire doesn't have a distinct culinary rep. It covers an enormous swathe of France, from the mountains of the Cevennes to the Bay of Biscay. But, as we have established, it's a river, which accounts for the menu's fishy leanings.

Head chef Ed Wilson has taken regional produce as a jumping-off point, rather than faithfully reproducing traditional recipes. There was an ascetic simplicity to a whole grilled mackerel, immaculately fresh – the skin glistening, the rich, dark flesh falling sweetly from the bone – which came naked to the table with nothing but an acidulous tangle of pickled cucumber. A heap of tiny brown shrimp tumbled from a mimosa fluff of grated egg white and yolk, with some silky, sour-sweet leeks vinaigrette. This is modern British cooking, rather than old-fashioned French.

From that day's menu we could also have feasted on clams with coco beans, sardines à la plancha, or lemon sole with seaweed butter. Earthier options included rabbit braised with salsify in Normandy cider, roast partridge with ceps and pickled walnuts, and an inspired partnership of girolles with sautéed artichoke heart and a single, shimmering egg yolk. How any of these dishes fit in with the Loire theme, I have no idea – but as we've established, the Loire covers a large area, so they could put pretty much anything on the menu apart from haggis, and we'd have gone along with it.

There has clearly been much fetishistic sourcing of ingredients. Take the star dish, coucou de Rennes, which showcased the charms of a breed of chicken apparently prized even more highly than poulet de Landes. Roasted and served in four substantial, skin-on tranches with a perfect still-life of pot-au-feu veg, the flesh was dense and characterful, the fragrant broth intense. At £18, it was an expensive introduction to this superior bird, but as Ed Wilson later ruefully told us, that's exactly what he'd paid for each of them.

As with Terroirs and its spin-offs, the big draw for many people will be the wine list, which majors on organic and biodynamic producers, but concentrates on the Loire and digs in deep. A pichet of Vouvray, the aptly-named Le Dilettante, served us well right through to the cheeses, a grand tour of obscurities introduced to us by our young French waiter with all the faraway reverence he might have brought to a line-up of legendary Gallic pop stars we'd never heard of.

With a couple of puddings – including an instant classic, tarte au vigneron, a boozy take on tarte tatin – we paid £50 a head, though it would be easy to have a fantastic time there and pay much less; the plat du jour, with a glass of wine, costs £10.

The room isn't particularly comfortable; long and narrow, and rammed with unforgiving spindly chairs and close-packed tables. But we still managed to draw out our lunch to nearly 4pm; as with Terroirs, the Green Man feels like some louche throwback, tempting you into a vertical tasting, followed by a horizontal recovery.

What it does have is personality, and that's a heart-warming quality to find in this chain-infested part of the West End. Some enlightened thinking on the part of planners and landlords has turned the tide in Soho and Covent Garden, and allowed quirky, independent little restaurants like this to open and flourish – lovely locals in the middle of the city. My lunch may not have taught me much about the Loire and its gastronomic tributaries. But I did feel as though I'd discovered another reason to fall in love with London.

Green Man and French Horn, 54 St Martin's Lane, London WC2 (020-7836 2645)

Around £50 a head, with wine

Food ****
Ambiance ***
Service ****

Tipping policy: 'Service charge is 12.5 per cent discretionary. All tips and service charge go to the staff'

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