Brown ground beef in 12-inch skillet over medium-high heat; drain. Add onion and cook, stirring occasionally, 2 minutes. Stir in Sauce, mustard and, if desired, salt and ground black pepper to taste. Simmer, stirring occasionally, 3 minutes or until heated through.
Turn into 2-quart casserole; evenly top with mashed potatoes.
Bake until potatoes are lightly golden, about 25 minutes.