Any kind of light fish—that is, cod, cusk, flounder, etc. Skin the fish by starting at the head and drawing down towards the tail; then take out the bones. Cut the fish into pieces about three inches square, and salt and pepper well. Butter such a dish, as you would use for escolloped oysters. Put in one layer of fish, then moisten well with sauce; add more fish and sauce, and finally cover with fine bread crumbs. Bake half an hour. The dish should be rather shallow, allowing only two layers of fish.
Heat the butter in a small frying-pan, and when hot, add the dry flour. Stir constantly until a rich brown; then add, gradually, the cold stock, stirring all the time. As soon as it boils, season well with salt and pepper, and then add the other seasoning. This quantity is enough for three pounds of fish, weighed after being skinned and boned, and will serve six persons if it is the only solid dish for dinner, or ten if served in a course.
Another way to serve fish au gratin, is to skin it, cut off the head, and take out the back-bone; and there are then two large pieces of fish. Season the fish, and prepare the sauce as before. Butter a tin sheet that will fit loosely into a large baking-pan. Lay the fish on this, and moisten well with the sauce. Cover thickly with bread crumbs, and cook twenty-five minutes in a rather quick oven. Then slip on a hot dish, and serve with tomato, Tartare or Hollandaise sauce poured around the fish.