Pizza Pasta Casserole

2 pounds ground chuck

1 large onion, chopped

2 28oz jars of spaghetti sauce

1 16oz package spiral pasta, cooked and drained

4 cups shredded mozzarella cheese

8oz sliced pepperoni

Brown ground chuck and onion; drain.  Stir in spaghetti sauce and pasta.  Transfer to 2 greased 13x9 baking dishes.  Sprinkle with cheese.  Arrange pepperoni over the top.  Cover and freeze one casserole for up to 3 months.  Bake the second casserole, uncovered at 350° for 25-30 minutes or until heated through.  To use frozen casserole:  Thaw in the refrigerator overnight.  Bake at 350° for 35-40 minutes or until heated through.