The Collaborative International Dictionary of English v.0.48:
Au gratin \Au` gra`tin"\ [F.] (Cookery) With a crust made by browning in the oven, especially by sprinkling with bread crumbs or grated cheese and browning in a broiler; as, spaghetti may be served au gratin. [Webster 1913 Suppl.]WordNet (r) 3.0 (2006):
au gratin adj 1: cooked while covered with browned breadcrumbs (and sometimes cheese)