Au gratin is a term familiar to most of us and we usually think of it as having cheese melted over the top of a dish and then browned.  In fact, the proper use of the term means to crumbs combined with a high fat product such as butter, cream, cheese or eggs for the topping.  The crumbs may consist of breadcrumbs, crushed potato chips, crushed crackers, or even corn flake crumbs. 

To prepare a dish au gratin, place the product in a dish, top with gratin and then place the dish under a salamander or in the oven, with a high heat, 475° to 550°F, until a golden brown crust forms.  Larger slices of product like meat or fish, and items with a high moisture content like tomatoes, must be done at a lower heat for a longer time to insure they have the proper doneness when they reach the right gratin brown