There can be precious few places where Jewish and Irish cuisines converge, but for both cultures, one dish is paramount. As St Patrick's Day approaches, the Irish are looking forward to a traditional festive treat Ashkenazi Jews enjoy year round - succulent salt beef, aromatic, prettily pink and unashamedly fatty.

It's almost certainly poverty that led the famine-prone Irish and the Jews of Eastern Europe to salt their beef as a way of preserving it, originally by rubbing cuts of meat with dry salt with grains the size of corn kernels. This is why Americans call salt beef corned beef, a name which invokes a much dryer and less palatable tinned product, popular in the era of austerity on this side of the pond.

Poverty would also have driven the choice of brisket for salting, a fatty shoulder cut which is tough unless cooked long and slow, when the fat melts into the sinews and makes the whole joint moist and tasty. These days, the salting is more often achieved by soaking the joint for four to seven days in a spiced brine before cooking, and silverside sometimes replaces the brisket. That pink colour comes from the addition of curing salt, and is not compulsory, though it does make the cooked beef look more appetising.

The days when really good salt beef could be picked up all over London's East and West End are gone - which explains the long queues at the Beigel Bake in Brick Lane, and the continuing popularity of the Brass Rail at Selfridges, both famous for their heaped sandwiches. But now there is a new kid on the block, who weekly sells out of salt beef even though, based in Southwark, he is far away from traditional centres of Jewish, or, indeed, Irish culture. Monty's Deli was born in the railway arch where Mark Ogus makes an appearance on weekends only to sell his home-made salt beef and pastrami - and foodie bloggers have reported needing to be there by breakfast time to be sure of getting a sandwich.

"At least it's better than when I only had a stall for one day, and now we've upped the quantities of meat we're curing so we can cater for the crowds until well into the afternoon," laughs Ogus, who is 35 but looks startlingly like a teenager. He must be unique in having made his reputation before getting the proper, week-round restaurant he longs for - he has already found fame on television with Tom Kerridge, on Radio 4's Food Programme and repeated praise from Observer food critic Jay Rayner.

Yet Ogus's adventures in salt beef are barely five years old. Growing up in Hatch End within a rich Jewish food culture - "I also make chopped liver, which is a combination of my mother's and grandmother's recipes" - he studied at the Chelsea College of Art, expecting to be a painter. "By the time I left there, I knew I wanted to be a musician, so I played with a rock and roll band instead."

During the excitement of being signed by EMI, making an album and touring Europe, Ogus got to the USA and found his destiny changing once again as he bit into giant deli sandwiches at Katz's in New York and Langer's in Los Angeles. After a spell in a Soho kitchen to learn how, he started salting his own beef, and running from his home in Bethnal Green to his parents' back garden on the other side of London to smoke some of it to create the pastrami which is an Ashkenazi speciality less known in this country: "It's essentially spiced and cooked with smoke instead of slow-simmered like salt beef."

Tweeting for premises led him to the now famous Maltby Street market in Southwark and his one-day stall selling salt beef and pastrami sandwiches. Fame soon followed, and a year ago, after teaming with chef Owen Barratt, who helped him develop more dishes, he expanded into a two-day share of a railway arch he has called Monty's Deli, in homage to his paternal grandfather, who adored salt beef. They make sandwiches of salt beef, pastrami, chopped liver and coleslaw all between the same two pieces of bread, a combination sandwich in true US deli style, as well as their own chicken soup, kneidlach, bagels, chopped liver and egg and onion. It's deeply traditional: "We render and use all our own schmaltz, and although Owen isn't Jewish, his chicken soup recipe is as good as my grandmother's."

Until this Sunday (March 8) Ogus also has a pop-up restaurant at Old Bengal Bar and Brasserie, New Street, in the City, but will be searching for permanent premises after that.

While he has experimented with both brine and dry-cure methods, another Jewish chef, Jason Freedman of the Minnis near Margate, is overwhelmingly in favour of dry-cure: "I tried wet-curing, but customers didn't like it nearly as much as the way we do it rubbed with salt and spices and left for 10 days before rinsing and air-drying." He then slow-cooks it in a water bath for 12 to 15 hours but says: "You can also cook a piece on top of the stove in 3½ hours at 98 degrees."

Both chefs' recipes are closely-guarded secrets, but Ogus's top tip is to buy the fattiest piece of brisket possible from a good butcher and wet or dry cure it for no less than four or five days before rinsing and cooking. Mustard or chrein are de rigeur to give the meat a bit of a kick, as is caraway-seeded rye bread for a sandwich base.

www.montys-deli.com