Peel and thinly slice potatoes. Toss slices in 1 cup water and 1/2 teaspoon cream of tartar. Drain. Put half of sliced potatoes in greased CROCK-POT. Top with half of onion slices, flour, salt and pepper. Add remaining sliced potatoes and onions. Sprinkle with remaining flour. Add butter and undiluted soup. Cover and cook on Low 7 to 9 hours. (High: 3 to 4 hours). Add cheese slices 30 minutes before serving (Recipe may be doubled for 5-quart model).