Pour sauce into slow cooker. If using my semi-homemade tomato sauce recipe, combine all sauce ingredients in the slow cooker. (If you don't have a slow cooker, just cook the meatballs according to the package instructions and combine with the sauce after they are done.)
Add meatballs and stir with a spatula to coat with the sauce.
Cook on high for 20 to 30 minutes. (Check after 15 minutes. If sauce is popping or splattering a lot, reduce to low.)
Cook pasta according to package instructions (start this before the meatballs are done to save time).
Preheat oven to 350°.
Stir pasta, sauce, and meatballs together. Stir in heavy cream.
Transfer half of the pasta mixture to a baking dish. Cover with half of the mozzarella cheese. Add another layer of pasta and cheese.
Bake for 20 minutes.
To reheat, add a small amount of water in the bottom of the baking dish and cover with foil. Bake for about 20-30 minutes at 350 degrees. If you microwave it, also add a small amount of water to the bottom of the dish so the food doesn't dry out.