Saute onion in oil 10 min or until soft. Stir in flour, sugar, paprika, mustard and salt; add water and vinegar slowly. Cook over med. Heat, stirring constantly, until thickened and bubbly. Combine potatoes, celery, green pepper, and sauce; spoon half of mixture into buttered 1 1/2-qt casserole. Arrange cheese slices over potato layer; top with rest of potato mixture. Bake at 350 degrees for 15-20 min or until salad is hot. Serve at once. 8 servings.