Au gratin refers to dishes sprinkled with bread crumbs or grated cheese, which is then browned in an oven or broiler. Gratin is sometimes used as a name for the brown crust that forms on such a dish.
Name origins
From the French au gratin, literally meaning "by grating." This refers to the original method of scraping bread crumbs from a pan and using it to top a dish after mixing with cheese.