Cut a large fish into slices and boil the trimmings in water to cover with a chopped onion, a little butter, and pepper and salt to season. Boil for fifteen minutes, strain, and simmer the sliced fish in it until done. Take up the fish carefully and squeeze the juice of three lemons into the liquid. Season with cayenne, take from the fire and add the yolks of six eggs and the whites of three beaten with a little cold water. Reheat but do not boil; pour over the fish and let cool. Serve very cold.