Put pieces of leftover turkey in a greased casserole. Add cooked peas, cooked onions, small can of mushrooms (reserve liquid), salt, pepper. Mix 1/2 cup mayonnaise, 1/4 cup flour, and two cups of turkey gravy, milk and reserved mushroom lisquid. Pour mayonnaise mixture over top of turkey, etc., in casserole. Top with buttered crumbs. Bake at 400 degrees for 20 to 30 minutes.