|Remove skin and bones from fish. Mash fish with fork. Remove crust from bread and soak slices in milk until quite soft. Squeeze out most of excess milk and beat bread to a pulp. Mix fish, bread, onion, parsley, and salt and pepper. Fold in stiffly beaten egg whites. Pour into greased baking dish, set in water, and cover mold with waxed paper. Bake 30 minutes at 375 degrees.
Remove paper, turn pudding onto hot platter and cover with Mushroom Sauce, or White Sauce to which capers have been added. Serve immediately.