Cook onion slices in butter until lightly browned.
Peel and slice potatoes. Place one fourth of potatoes in bottom of greased 2-qt. baking dish. Add one fourth each: onion slices, cheese, flour, salt and pepper. Repeat layers. Pour milk over top. Sprinkle with crumbs.
Cover and bake in moderate oven (350°) 1 hour. Remove cover the last 15 minutes.