In greased casserole, alternate squash and onions with mayonnaise mixture, ending with mayonnaise mixture. Sprinkle buttered bread crumbs on top. (For buttered bread crumbs, use 2 slices bread and 1/4 cup butter.) Put bread crumbs in blender to crumble. Put in pan with butter to mix. Bake casserole 30 minutes at 350 degrees.