Slice the mushrooms and arrange a layer in a very thickly buttered fireproof dish. Season well. Put a layer of crumbs and repeat layers alternately, ending with a layer of crumbs. Dot each crumb layer with butter, and put plenty on the top. Cook in a moderate oven, 350° F., gas mark 4, for 30–35 minutes. Make sure the mushrooms are tender, and brown the top under a hot grill if necessary.