Soak the mackerel for twenty-four hours, then lay it in a shallow stew-pan, and cover with milk or cream. Simmer for fifteen minutes. Remove the fish carefully, and place it on a hot dish. Add to the milk or cream in the stew-pan one tablespoonful each of butter and flour rubbed together. Stir until a little thickened, and the flour cooked; add a little pepper and chopped parsley, and pour the sauce over the fish.