This question came up on another board I visit and I thought I'd ask it here.
The knish (the k is pronounced) is one of NYC's little treasures. Traditionally, knishes come in two shapes and two flavors. The best are round and baked. These come filled with kasha (buckwheat) or potato. The potato variety also contains finely chopped onion and, if you're lucky, a bit of schmaltz (chicken fat).
In my opinion, the square, fried variety is far inferior. This is the kind you find frozen in the grocery or on the average food cart. Most of them both look and taste like throw pillows.
Although Yonah Schimmel is said to be the sine qua non of knishes, We preferred the late, lamented Mrs. Stahl's in Brighton Beach. That place used to make a Spanish rice knish. It also offered cocktail sizes and dessert knishes filled with fruit.