Gratin aapted from French cuisine is a type of casserole dish that is covered with Béchamel sauce or Mornay sauce, topped with buttered breadcrumbs or grated cheese and either baked or broiled, then served in its baking dish with a golden crust. Cooking au gratin is a technique rather than exclusively a preparation of potatoes (which is specifically a gratin dauphinois): anything that can be sliced thin, layered with a cream sauce and baked is material for a gratin: fennel, leeks, crabmeat, celeriac, aubergines.