Gratin adapted from French cuisine^[1] is a type of casserole dish that is covered with Béchamel sauce or Mornay sauce, topped with buttered breadcrumbs or grated cheese^[2] and either baked or broiled, then served in its baking dish with a golden crust.^[3] 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.

Potatoes gratiné is one of the most common of gratins and is known by various names. In the East Coast and midwestern United States, the dish will be referred to as scalloped potatoes. (Note that the term scalloped originally referred to a seafood dish rather than to a scallop) ^[4]. In English-speaking Canada, the dish will be referred to as au gratin style potatoes. In French-speaking Canada, the dish will be referred to as pommes de terre au gratin. The dish may also be known as gratin dauphinois, pommes de terre dauphinoises or potatoes dauphinoises.

Potatoes and onions au gratin with anchovies are as traditional in Swedish cuisine as they are in French.^[5]

The name Gratin dauphinois refers to the Dauphiné region of France, where this method of preparing potatoes is a specialty. The ingredients composing a typical gratin dauphinois are thinly sliced and layered potatoes, milk (and/or cream or creme fraiche), cheese, garlic, salt and pepper. Eggs may sometimes be mixed with the milk/cream. Gratin savoyard is a variation found in the neighbouring region. Unlike gratin dauphinous, it is made without milk, cream or eggs. Instead, it consists of alternating layers of sliced potatoes and Beaufort cheese with bits of butter, and bouillon is poured over before baking.^[6]

• Lyonnaise potatoes