A knish /ˈknɪʃ/ or knysh is an Eastern European snack food consisting of a filling covered with dough that is either baked, grilled, or deep fried.n the most East European traditional versions, the filling is made entirely of mashed potato, ground meat, sauerkraut, onions, kasha (buckwheat groats), or cheese. Other varieties of fillings include sweet potatoes, black beans, fruit, broccoli, tofu, or spinach.