Cook squash, covered, in boiling salted water 20 minutes or until tender. Drain and mash. Drain again, if necessary.
Combine squash, 2 tablespoons butter, onion, flour, salt, pepper, and milk; stir well. Pour mixture into a lightly greased 1-quart casserole.
Combine breadcrumbs and melted butter; spoon over squash mixture. Bake, uncovered, at 350° for 30 minutes or until lightly browned.