This is a favorite manner with the French of cooking fish.
The fish is served in the same dish in which it is cooked. It is called a gratin dish - generally an oval silver-plated platter, or it may be of block-tin. A fish au gratin is rather expensive, on account of the mushrooms; however, the French canned mushrooms (champignons) are almost as good as fresh ones, and are much cheaper.
First put into a saucepan butter size of an egg, then a handful of shallots, or one large onion minced fine; let it cook ten minutes, when mix in half a cupful of flour; then mince three-fourths of a cupful of mushrooms. Add a tea-cupful of hot water (or better, stock) to the saucepan, then a glass of white or red wine, salt, and pepper. After mixing them well, add the minced mushrooms and a little minced parsley. Skin the fish, cut off the head and tail, split it in two, laying bare the middle bone; slip the knife under the bone, removing it smoothly. Now cut the fish in pieces about an inch long. Moisten the gratin dish with butter, arrange the cuts of fish tastefully on it, pour over the sauce, then sprinkle the whole with bread-crumbs which have been dried and grated. Put little pieces of butter over all, and bake. The dish may be garnished with little diamonds of fried or toasted and buttered bread around the edge. Or,
This is a pretty dish au gratin: Put mashed potatoes (which must be still hot when arranged) in a circle on the outside of the gratin dish, then a row of the pieces of fish (which have been cooked as just described) around the middle of the dish, or just inside the potatoes. Put some mashed potatoes also in the middle of the dish. Garnish here and there with mushrooms. Pour the sauce just described and bread-crumbs over the fish, and bake five or ten minutes.