Cheddar Burger Mashed Potato Bake

2 lb ground beef
1 medium onion chopped
1 jar Ragu Cheese Creations-Double Cheddar
2 tsp dry mustard
4 C prepared mashed potatoes

Preheat oven to 425. In 12" skillet, brown ground beef; drain. Add onion and cook 2 minutes. Stir in cheese sauce, mustard and salt & pepper to taste. Simmer until heated through. Pour into 2qt casserole. Top with potatoes. Bake 25 minutes or until lightly golden.