Classic Potatoes Au Gratin

This is very creamy and really cheesy.

4 cups potatoes, peeled, cooked (fork tender), and thinly sliced

1/2 pound (8 ounces) Velveeta

1/2 cup onion, chopped

1 teaspoon dry mustard

1/4 teaspoon pepper

Preheat oven to 350 degrees. 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.