Potato Casserole with Creamy Cheese Sauce

Sliced potatoes are baked with a creamy cheddar cheese sauce.
Ingredients:
3 tablespoons butter
1 tablespoon minced onion
3 tablespoons flour
1 teaspoon salt
1/4 teaspoon pepper
1/2 teaspoon dry mustard
1 1/2 cups milk
8 ounces grated American or mild Cheddar cheese
4 cups cooked diced potatoes
1/4 cup soft bread crumbs
1 tablespoon melted butter
Preparation:
Melt 3 tablespoons butter in saucepan.
Add onion and cook until tender.
Remove from heat.
Mix together the flour, salt, pepper, and dry mustard.
Add flour mixture to the butter and onion, stirring until smooth.
Gradually add milk, stirring constantly.
Cook over low heat, stirring constantly, until thickened.
Add cheese, saving about 1/2 cup for the topping.
Continue cooking slowly, stirring constantly, until cheese is melted.
Remove from heat.
Put potatoes in a lightly greased baking dish.
Pour sauce over potatoes.
Sprinkle bread crumbs combined with 1 tablespoon of melted butter over the top.
Top with the reserved cheese.
Bake at 350 for 20 minutes, or until lightly browned.