White Sauce:
3 Tbls butter or oleo
3 Tbls flour
3 c milk
salt and pepper

6-8 cooked potatoes, cubed
2 c Grated American cheese
1 Tbls onions, minced fine

Melt butter, blend in flour, then milk.  Cook stirring constantly until thick and smooth.  Season to taste. 

In buttered baking dish, arrange layers of potatoes, white sauce, grated cheese and small amounts of minced onion.  Repeat until dish is full.  Sprinkle with grated cheese. 

Bake 350 about 45 mintues