Chicago and Cleveland serve the best corned beef. Montreal has the best smoked meat, New York owns pastrami, and no one can touch London for pickled tongue.

That’s what 30-year-old Toronto native David Sax says, and he should know: he spent three years eating his way through more than 150 Jewish delis to research Save the Deli: In Search of Perfect Pastrami, Crusty Rye and the Heart of Jewish Delicatessen.

Mr Sax’s book, newly released in the US, is a delicious romp through a fast-disappearing world.

“The Jewish deli is dying,” he said. “Each time I hear a deli closes, something inside me dies.”

In 1931, there were 2,000 Jewish delis in New York City; today there are less than 20. Mr Sax believes there are now just a few hundred worldwide, most in the US.

Mr Sax chronicles the history of the deli, from when German immigrants brought the cuisine to New York in the 1820s. By the 1870s and ’80s, German Jews had made kosher modifications to the treif recipes: schmaltz instead of lard; ptcha, or jellied calves’ feet, instead of pig trotters.

He describes the “kosher-style” deli, an American innovation that originally differed from its kosher counterpart mainly in hours of operation (they did not close on the Sabbath) and lack of rabbinical supervision.

In the 1950s and ’60s, the kosher-style deli eventually succumbed to popular tastes, serving cheese on turkey sandwiches, with ice cream for dessert.

But most of this book is a paean to the gloriously fatty Ashkenazi cuisine that is the signature of the Jewish deli: cabbage rolls in sweet-and-sour tomato sauce; matjes herring; corned beef pickled in vats of brine; pastrami rubbed with secret spice mixtures, then smoked and steamed to perfection.

Mr Sax spends most of his time in North America, but stopped off in London long enough to slam Blooms, where he ate what he called “one of the more appalling deli meals of my life”, and to kvell over the hand-carved tongue at B&K Salt Beef Bar.

He found London delis more upscale than their American counterparts.

“While deli customers in North America want their delis down, dirty and cheap, London’s delis have adapted to the city’s refined opulence.”

In his enthusiasm, Mr Sax glosses over the real possibility that the Jewish deli may not survive outside a few cities. The only place where the deli scene is thriving, he claims, is Los Angeles.

“The delis out there are bigger, are more comfortable, and serve better food than any other city in America.”

Mr Sax’s mission to save the deli depends on young Jews going into the business. He insists the market is there, as a new generation looks back nostalgically to a cuisine that represents a simpler, more comforting era.

“People aren’t really looking for innovation in deli,” he said. “The best things I see in the new delis are a return to the way our grandparents cooked. It’s ‘innovative’ today to pickle your own meat or make your own kishke.”