What is a gratin?
In French, the word gratin originally referred to the tasty crust left behind in a pan after baking, which was scraped off and eaten as a bonus for the chef.
Now, cooking something au gratin can mean preparing a dish that is cooked from scratch in the oven, like a gratin dauphinois made with potatoes and cream; or combining several cooked ingredients in casserole form, like a baked pasta dish.
A gratin is usually topped with grated sharp cheese and/or breadcrumbs. Adding just cream will also produce a lightly browned crust if baked in high heat. Technically, macaroni and cheese is a gratin, in that a protective crust forms while it bakes, giving it a lightly browned and crunchy topcoat, while keeping the rest of the dish moist.
Gratins are served straight from the dish; hence the term gratin dish, which refers to an (often oval-shaped) oven-safe serving pan. That old-fashioned favorite, onion soup gratinée, is made by pouring soup into oven-safe tureens, topping them with toasted bread and grated cheese, and baking au gratin until gooey.
In French, le gratin is also an idiomatic expression meaning "the upper crust" of society.
Recipe: Asparagus-Fontina Gratin
Recipe: Portobello Gratin
Recipe: Pak-Choi Gratin (Experimentation of Taste)
Recipe: Cardoon Gratin (Cooking with Amy)