Pare and thinly slice potatoes. Toss slices in water and cream of tartar; drain.
Put half of sliced potatoes in greased slow cooker.
Top with half of onion slices, flour, salt and pepper.
Add remaining sliced potatoes.
Top with remaining onions and flour.
Add butter and undiluted soup.
Cover and cook on low 7-9 hrs or high 3-4 hrs.
Add cheese 30 minutes before serving.