Classic Potatoes Au Gratin

4 cups potatoes, thinly sliced (I peeled mine, the cookbook directions did not)

1/2 pound (8 ounces) Velveeta

1/2 cup onion, chopped

1 teaspoon dry mustard

1/4 teaspoon pepper

Preheat oven to 350 degrees.  Cook potatoes in boiling water for 8 to 10 minutes or until fork-tender.  Drain.  Toss potatoes and remaining ingredients in 2 quart casserole; cover.  Bake for 22 to 25 minutes or until potatoes are tender and cheese is melted.  Stir before serving.