A casserole is a baked dish for many different types of food, mixed together in a fluid based on liquid. The start ingredient is often a roux. Often, one of the ingredients is a can of soup. The whole point of a casserole is that it cooks material slowly. That means it can use ingredients which would not be edible if fried or roasted. The casserole needs to simmer below boiling point for hours, depending on the ingredients. That way tough meat and vegetables become soft and edible.

It is a simple way to cook a nutritional meal.^[1]

Casserole is from a French word meaning "sauce-pan"; a large, deep dish used either to cook something in an oven or to serve the food cooked in it. The French word  "casserole"  had been derived from the old Provençal word, "cassa" and the Medieval Latin word, "cattia", both of these words meaning "ladle".

Casseroles are usually cooked slowly in the oven, usually covered. They may be served as a main course or a side dish, and may be served in the vessel in which they were cooked. Types of casserole include ragout, hotpot, cassoulet, tajine, moussaka, lasagne, shepherd's pie, gratin, rice or macaroni timballo, and carbonnade.^[source?]

A popular type of casserole is green bean casserole, which has green beans, milk or cream, and fried onions. It was first cooked in Berlin, New Hampshire in 1866 by, Elmire Jolicoeur.

The second illustration proves how old the idea is. That casserole dish has a lid, and a spout for steam to escape. The heat is applied from below as it is with a stew.

Early casserole dishes were rice-based that were pounded and pressed and then filled with meat. The first popular meals included French cassoulet, British pot pies, Italian lasagna, and Greek moussaka. Macaroni and cheese is the oldest written casserole recipe found in 1250.