Six-13 may be named after Judaism's astonishing number of dietary laws, but it has nothing against great tasting food

Six-13 19 Wigmore Street, London W1; tel: 020 7629 6133

Six-13 19 Wigmore Street, London W1; tel: 020 7629 6133

Open Monday, Thursday and Sunday for lunch and dinner, Friday for lunch and Saturday for dinner

Jonathan's birthday falls on a Friday but we're going out on the Thursday. His starter for 10: Why? "Give me a teeny tiny clue," he says as I drive him towards the neon twinkle of Oxford Street.

"There's a very good reason," I say. "And it'll make you laugh."

"Mmmm, Thursday...?"

"It's your Birthday Eve. Isn't that exciting enough?"

"Aha!" he says then, "I think I've got it -"

"You couldn't have."

"It's somewhere," he says with a sideways look, "that isn't open tomorrow -"

"Maybe..." I say (worried).

"Because in deepest, darkest winter, Friday night is... the Sabbath?"

"Oh, great," I say, knowing how Dr Watson must have felt.

He laughs. "I cracked it in the shower. I've been toying with you since, you were so enjoying the suspense. Are we nearly there?"

We're looking for a parking place in the one-way maze between John Lewis and the BBC, stamping ground of opticians and prosthesis makers. It seems a funny place for this newish, happening restaurant that combines the 613 Biblical dietary laws with British nouvelle. Or maybe not - just up from the wan, 1980s crêperies of St Christopher's Place, the cardigan-and-taxi façade of the Wigmore Hall and, much more importantly, the pharmaceutical sweetie shop that is John Bell & Croydon (home of diaphragms in all the weird sizes - trust me, I know).

The front looks like an undertaker's. OK, a very upmarket, discreet, marble-and-Mafia, joyously baroque undertaker. But inside it's all austere 1930s elegance and monochrome - starched linen tablecloths, no pictures on the pristine cream walls, lots of black hair, black jackets, a smattering of black yarmulkes. My dress is cerise and I stand out like a cherry on a chocolate cake.

The people on either side of us are discreet, nervous, old-ish, laden with gold jewellery, and stuck for small talk. For all this place's apparent comfort and thoughtful charm, the tables are far too close together - so close that you'd be sorely tempted to ask your neighbour to pass the salt or even (as chez Myerson) to reach bluntly across him for it, were the food to need it.

Happily, it doesn't. It's pretty much fantastic (if not cheap).

My partner's father was Jewish but he met a shiksa (working in DH Evans, just round the corner, as it happens) and married out. So now Jonathan merely has the nose and a taste for the Marx Brothers and matzos. I just pinched the name.

Jonathan says the list of starters are "everything my father liked best to eat - weird to see them all listed in one place". He died six months before J and I got together. Known fondly by our children as Our Big Dead Grandfather (6ft 5in stocking feet), I miss him for the benign grandfather I know he would have been. Tonight I discover that he liked: hickory smoked salmon, rollmop salad with shallots and capers, barley soup with beef and butter beans.

In homage to his dad, Jonathan has the rollmops. In homage to my sore throat, I have the chicken soup. Both are faultless - especially his herring, which has all of the wide-awake, velvety tang and none of the fishy acidity of the ones in supermarkets.

His sea bass and my roast duck breast are equally good. The dark, plump meat of duck is gloriously coupled with the purple attack of cabbage. But the home-cut chips are The Best Chips I've Had Anywhere Ever, Including Southwold Beach. Pale brown on the outside, meltingly white on the inside, with that slight, glutinous, desirable fattiness around the edges.

By this point, I am pleasantly drunk on the Pouilly Fumé and I feel I could go on eating for ever. This combination of dietary laws and modern cuisine somehow works a Moses-like miracle. So I manage a coconut rice pudding with mango jam and, still on his race-memory kick, Jonathan has lockshen pudding with fresh summer berries. It's like fritters put through a Play-doh machine.

We attempt a brief, tired snog outside John Bell & Croydon, and I ask him if he's enjoyed his special kosher birthday.

"But even my grandfather ate bacon."

"He didn't!" I'm shocked. "How did he get away with it?"

"When he came round for a meal. My mother made this bacon and tomato soup. He pretended it was beef and tomato."