Au gratin is one of many fancy-sounding French culinary terms to enter English during the 19th century, when a taste for French food, especially among the wealthy, developed in America. When you cook something au gratin or simply gratin, you are not, in reality, doing anything fancy at all. In fact, the verb “to gratinée,” as used today, means simply to give a dish a golden brown top.
The word gratin is given many alternative and incorrect definitions. Since most gratin dishes, such as potatoes au gratin, use plenty of cheese, many people think that the term is French for “with cheese.” However, it is possible to make a dish au gratin without cheese.
Other sources say that gratin or au gratin refers to a dish with a browned topping, whether breadcrumbs, cheese, or a combination of both. Some even claim that gratin is a French word for a baked casserole.
The word gratin actually derives from the french word grater or gratter, meaning “to grate.” It would be tempting, then, to think that gratin refers to grated cheese. But, this is not what the word originally referred to. Instead, it meant something more like “scrapings.” This referred to the browned crusty material that forms on the bottoms and perhaps to the act of scraping loose these crusty bits and stirring them back into the dish during cooking. It now tends to refer to the browned crust that forms on the top of a baked dish, whether this crust forms by itself or is accelerated by passing the dish under a broiler.
There are those who insist, then, that gratin always refers to a topping of breadcrumbs and/or cheese that is browned and crisped, forming a nice crust. However, there need not be any additional toppings added to a dish for it to be au gratin. The famous dish gratin dauphinous, for example, sliced potatoes baked in cream, has no topping added.
The original poulet au gratin, as well, had no cheese or breadcrumb topping. It was simply a roast chicken served on top of a layer of baked forcemeat. Today, this dish refers to pieces of chicken covered with a sauce and usually sprinkled with cheese which is browned in an oven or under a broiler.
The term gratin is sometimes used to refer to a forcemeat for which the main meat is seared before grinding.
The word gratin is also used to identify the types of cookware in which the dishes are cooked. These dishes, such as these HIC Au Gratin Baking Dishes, are traditionally oval, but can also be round, and come in graduated size variations. The traditional material used is clay-based ceramic but they can also be metal or oven-proof glass.