Ondine, 2 George IV Bridge, Edinburgh

I've visited Edinburgh many times over the years, for work and play. I've experienced the cultural highs of the Festival and some memorable lows (the world premiere of Tubular Bells 2 at Edinburgh Castle springs to mind). And though I've often eaten well, I've never eaten dazzlingly well. Edinburgh has plenty of good restaurants but great ones are elusive, particularly at the upper end of the market, where too many of the famous city-centre names prove to be disappointing tourist traps.

This year, though, something seems to be happening in Edinburgh. There's a new excitement around the dining scene, and a broadening of gastronomic horizons. My Edinburgh sources (ooh, get me, Deep Throat) bombarded me with recommendations when I told them I was visiting. New brasseries from Tom Kitchin and Martin Wishart, the capital's most celebrated chefs. A fab gastropub, The Dogs. Wedgwood, a secret gem adored by locals. There was even an honourable mention for the blingy Hotel Missoni, where hunky doormen sport op-art Missoni-striped kilts, and the clientele is pure (Forth) bridge and tunnel.

It was Ondine, though, that emerged as the most popular suggestion. Not strictly new (it opened in 2009 ) and hardly a secret gem – the Good Food Guide declared it Scottish Restaurant of the Year 2011 – it has largely escaped the attentions of the London-based reviewing mafia.

On paper, Ondine has a huge amount going for it. It's a seafood restaurant, in a city with a proud, but neglected tradition of them. The chef/proprietor, Roy Brett, once oversaw Rick Stein's Padstow empire, and his passion for all things fishy is matched by a commitment to sustainable sourcing. And most importantly for the festival-goer, it's central. So central, you could throw an oyster shell out of the window and hit a minor royal processing up the Royal Mile, or hear the cannon-fire from the Castle's nightly Tattoo (or, spooling back to 1992, the bagpipe chorus from Tubular Bells 2).

What we weren't prepared for, though, given its Old Town setting, was Ondine's appearance. The restaurant is on the first floor of a shiny new faux-vernacular development, apparently made from giant caramel Lego bricks, incongruous amid all that ancient soot-blackened stone.

Nor does the dining room offer the kind of Old Town grandeur we were expecting; less Conran-style brasserie, more like the restaurant in a new-build business hotel. A huge horseshoe-shaped bar dominates the room and breaks up sightlines, and the place feels cramped and low-ceilinged.

The black-and-white colour scheme with clashing monochrome textiles looks chilly and dated. "Like one of the best restaurants at Heathrow," as my dinner-date, Harry, put it.

The menu, though, is a lot more colourful, an international greatest hits collection of fish dishes which could be marketed as Now That's What I Call Seafood, Volume 1. Oysters and crustacea galore; salt and pepper squid tempura; lobster thermidor; mouclade of Shetland mussels; roast sea bream curry. And there's meat too – any meat you like, so long as it's steak tartare or sirloin.

To get the full experience, we ordered the roast shellfish platter (£38) to share as a starter, although I was secretly feeling I couldn't get too excited about another plate of shellfish. And then it arrived, a fabulous selection, garlanded with samphire, chilli and sea purslane. Local lobster, crab and langoustines, grilled with butter and wild garlic, whose warm buzz set the tastebuds singing. Razor clams, cockles and mussels, cooked marinière style, in white wine. Loch Fyne oysters supplying a blast of iodine freshness. This was a feast, and by the time we'd worked our way messily through it, I'd fallen in love with shellfish all over again.

Our young waiter was indulgent in the face of the debris-spattered disaster area that was now our tablecloth. "Don't worry, I've seen this kind of thing before," he reassured us, like a paramedic at an accident scene.

A main course salad of grilled squid, black olives, hunks of cucumber and feta was the kind of dish you dream of getting on a Greek holiday, but never do. Grilled whole plaice, removed from the bone, came with anchoïade and a slippery sour-sweet tangle of red pepper. Both dishes were absolutely huge.

Small wonder that the place was hoaching on a Friday night. As the daylight faded, the décor began to look less stark, the arriving clientele more glamorous, and the room gained some of that missing atmosphere.

A shared dessert – chocolate mousse holding a warm pool of salted caramel – and a £38 bottle of Alabarino brought our bill to around £65 a head, but then we did splash out, in every sense. You could simply perch at the bar for a quick pre-show two courses, or post-show plate of oysters. Though on the strength of our feast at Ondine, I can heartily recommend the full, forget-the-show, festival-what-festival? three courses.

Ondine, 2 George IV Bridge, Edinburgh (0131 226 1888)

Food
Ambience
Service

Around £40 a head before wine and service. Lunch/ pre-theatre menu £16.95 (2 courses) or £19.95 (3 courses)

Tipping policy: "Service charge is discretionary. All tips and service charge go to the staff"

Side Orders: Edinburgh eateries

The Honours

Martin Wishart's new Edinburgh brasserie specialises in such classics as rabbit à la moutarde for a reasonable £16.95.

58a North Castle Street (0131 220 2513)

The Dogs

David Ramsden's sensational gastropub serves up Brit-inspired food – try the devilled liver, onions, bacon and mushrooms on toast.

110 Hanover Street (0131 220 1208)

Wedgwood

Wild Scottish deer with poached pear, carrot and olive oil purée, red wine and anise jus is typical of the inventive cooking here.

Royal Mile, 267 Canongate (0131 558 8737)