Gratin is a cooking technique that involves topping a mixture of ingredients with a crust (which often features cheese, breadcrumbs, and egg or butter) inside a shallow baking dish. As this cooks, a brown and crunchy crust forms on top and the inside of the gratin recipe gets bubbly and warm. By this definition, Baked Mac and Cheese to Chicken Tetrazzini also qualify, but creamy vegetables and tubers are often the star in gratin casserole recipes.