The knish belongs on a sandwich shelf only by an honest stretch: it is not bread around a filling but filling wrapped in dough, and the New York version is most often eaten as a dense brick in hand rather than split and built. What makes it sandwich-adjacent is the one move that turns it into one: the square knish is slit along an edge, opened like a pocket, and a smear of deli mustard is worked into the seam. That slit is the whole sandwich argument. Closed, it is a baked dumpling; opened and dressed, it reads as a stuffed pocket eaten the way a sandwich is.
The craft is in the ratio of shell to fill and in the bake. The filling is the point, most classically seasoned mashed potato, sometimes kasha or ground meat, dense and well salted so it holds together when the dough is cut. It is sealed in a thin dough that has to be strong enough not to split under a heavy, soft interior and tender enough to still read as a wrapper rather than a crust. The square Brooklyn style bakes thin-skinned and large; the round Yiddish-counter style runs a thicker, twisted dough. Either way the bake time is set by the shell, and the filling goes in cooked, because nothing inside will finish in the oven. There is no acid and no crunch built in, which is exactly why the mustard matters: worked into the slit, it is the single sharp counter to an otherwise soft, starchy, one-note interior, the deli's standard fix for a rich filling that has nothing bright in it.
The variations are mostly the filling and the dough. Potato is the baseline; kasha is the buckwheat reading; spinach, sweet potato, and meat versions swap the center while keeping the sealed-pocket logic. The bialy and the boiled-and-baked bagel share the same appetizing counter and the same impulse toward a sturdy carrier for a heavy load, and those relatives deserve their own articles rather than being crowded in here.