‘a covered heat proof vessel in which food is cooked and served’ (NSOED) or, by extension, the food cooked in such a vessel. The word has a complicated history, starting with a classical Greek term for a cup (kuáthos), progressing to a Latin word (cattia), which could mean both ladle and pan, then becoming an Old French word (casse, via the Provençal casa), which then became cassole (diminutive cassolette and thence, too, the dish cassoulet) and casserole. Besides explaining this,
When English took it over from French at the beginning of the eighteenth century it meant a dish of cooked rice moulded into the shape of a casserole cooking pot and then filled with a savoury mixture, say of chicken or sweetbreads. It was also applied by extension to a border of rice, or even of mashed potato, round some such dish as fricassee or curry: Mrs
Beeton’s recipe for a ‘savoury casserole of rice’ describes such a rice border. Then some time around the 1870s this sense of casserole seems to have slipped imperceptibly but swiftly into a ‘dish of meat, vegetable, and stock or other liquid, cooked slowly in the oven in a closed pot’, its current sense.