Au gratin signifies that the fish is gratinated or browned like toast on the surface, and therefore, that it is covered with bread crumbs. It comes handy to express it in that way, as the fish is not exactly breaded as for frying.

Split the fish in halves and dredge both sides with salt and pepper. Put a spoonful of drippings into your baking pan and let it get hot. Dip the skin side of the sides of fish in either milk or egg, and then in cracker meal or crumbs ana place in the pan with the breaded side up. Bake it brown and baste once with butter. Divide neatly in pieces with a sharp knife. Serve either sauce, gravy, or potato balls with it.