Potatoes Au Gratin ( Elsie)

4 or 5 large potatoes, cooked, peeled and cold
Slice cold potatoes into a shallow greased baking dish.
Sauce
1/4 cup butter or marg.
1/4 cup flour
1/4 tsp salt
1/8 tsp pepper
1 cup milk
8 slices american cheese slices
1 tsp grated onion
Melt butter in a saucepan over medium heat. Stir in flour, salt and pepper; stirring constantly until mixture is smooth.
Add milk slowly,stirring constantly until well blended. Add grated onion, and cheese slices 2 @ a time stirring until melted. Repeat until all cheese has been added.
Pour sauce over potatoes and bake @ 350 for 45 minutes.