Sauce:
2 T. Butter
1/2 C. Flour
2 C. Milk
2 C. Cheddar cheese
Recipe
Melt butter in saucepan. Add flour and milk, whisk. Add cheese. Cook til thickens up and cheese has melted.
In 9x13 pour a little cheese sauce, sliced potatoes, diced onion. Repeat once but leave enough cheese sauce to top with. Bake uncovered at 350 for 1 hr.