Classic Potatoes Au Gratin

1/4 cup butter
1/4 cup flour
1 tsp salt
1/2 tsp dry mustard
1/4 tsp pepper
2 1/2 cups milk
1 pkg (8 oz) shredded cheddar cheese
2 Tbsp chopped onion
6 cups thinly sliced peeled potatoes

Preheat oven to 350.  Melt butter in saucepan.  Blend in flour, salt, mustard and pepper.  Gradually add milk, stirring until well blended; cook until thickened, stirring constantly.  Add 1 1/2 cups cheese and onion;  cook until cheese is melted, stirring frequently.  Layer potatoes and cheese sauce alternately in 2 quart casserole dish, ending with a layer of the cheese sauce.  Bake 1 hour and 15 minutes or until potatoes are tender.  Sprinkle with the remaining cheese.  Bake an additional 5 minutes or until cheese is melted.

Serves 8