The first Jewish delis, which appeared with the arrival of the German Jews in the middle of the 19th century, were a kosher adaptation of German delis. Beef was substituted for pork in the sausages and cured meats and dairy products were verboten, banished to a separate category of stores, the appetizing. They usually had tables and often waiters and they carried fewer grocery items than their German counterparts.
Arthur Schwartz writes that the German-style food they offered would have been unfamiliar to most of the Eastern European Jews who began arriving in the city in large numbers later in the 19th century but they were the only places where they could find kosher prepared foods. While some of the new immigrants who had come from rural shtetls might have found deli food a novelty, those who came from towns and cities in Austria-Hungary, Rumania and Poland with German or Germanicized communities might well have encountered deli food before. Delis were also a welcome convenience for tenement dwellers with rudimentary kitchens. As time went on kosher delis began to add Eastern European Jewish foods like Romanian pastrami, based on Turkish basturma, a spice-cured and air-dried beef; soups like barley with wild mushroom, split pea and sweet and sour cabbage borscht; knishes filled with kasha or mashed potatoes and the ultimate Eastern European Jewish comfort food, chicken soup. The meats had a lot of garlic. Elliot Willensky wrote that you could smell the garlic in the meats of a kosher deli at twenty paces. Later as they moved into more upwardly mobile neighborhood, the sit-down delis added steaks and chops (a skirt steak was known as Romanian tenderloin). They usually also had roasted chickens and fresh roasted turkey to take out or eat in as well,
If you ate in, the grumpy or sarcastic waiter, usually an older man, put the cucumber pickles (half sours and full sours), cabbage slaw and pickled green tomatoes on your table before asking for your order. Franks were popular items, served grilled on a toasted bun with Heinz vegetarian baked beans or spicy brown mustard and sauerkraut but not with the onions in tomato sauce you found on hot dog carts. Schwartz says that while frankfurters were a German invention it was the Jewish deli that popularized them in New York.
Kosher delis all looked pretty much the same. Ellioy Willensky offered a detailed description of a typical Brooklyn kosher deli in When Brooklyn Was the World. The window had a neon sign with Hebrew lettering and underneath the name of the brand of cold cuts that were served. Just inside, visible from the street, was the frankfurter grill where franks and knishes were cooked. A little further inside were the “specials” ( knockwursts). Adjacent was a mustard bowl and a canister of steaming sauerkraut. Inside was a small serving bar, which held the cash register and sometimes a beer spigot and short length of brass rail. Cold cuts cut on a bias were displayed in glass cases, including hard salami, soft salami, chicken salami, roast beef, rolled beef, corned beef, bologna, brisket and tongue. The Rumanians added pastrami. On top of the meat case there were small knobs of thickly sliced knublvoorsht (garlic wurst) with a hand-lettered sign “A nickel a schtickl.” A taller case held cole slaw, potato salad, chopped liver and green cans of Heinz baked beans with the U from the Union of Orthodox Rabbis certifying it as kosher. Red peppers, green tomatoes and a wide variety of pickles were stored in large jars. Mimi Sheraton writes that the pickles were often in barrels. The perfect deli pickle was plump, pimpled, dark green and almost white inside according to Willensky.
Most people ordered sandwiches, often to go, which was cheaper than eating in. The order came with tapers of mustard wrapped in little squares of buff-colored wax paper. The choice of bread was limited to rye or “club,” which were slices of Italian bread. Mimi Sheraton wrote that the her family usually ordered salami, tongue and eggs, franks and beans, or sandwiches of rolled beef, pastrami or corned beef, served with dill pickles. Before the advent of non-dairy creamers, kosher delis usually did not serve coffee. Adults washed down their meal with tea with lemon or Dr. Brown's Cel-Ray thought to be a restorative that cut through the grease and garlic. Since milk was verboten in a true kosher deli, the kids would have soda, perhaps Dr. Brown's cream, black cherry or ginger ale. Even some Jewish families who did not keep kosher found the mere idea of drinking milk with meat stomach-turning. You would not find cheesecake at a true kosher deli. For that you had to go to a dairy restaurant.
By the 1940s there were also a number of "kosher-style" delis, which served kosher foods and generally kept kosher food rules, but were not under rabbinical supervision. There were also a number of restaurants like Lindy's that served Jewish deli food but were not at all kosher. That's where you could get salami and cheesecake or corned beef with swiss cheese.