Put the frozen meatballs and marinara (or spaghetti) sauce in a greased or lined slow cooker and stir to coat.
Cover and cook on low for 6-7 hours or on high for 3-4 hours or until heated through.
Preheat the oven to 350˚F. Slice the hoagie buns. Put two half-slices of mozzarella and pepperoni slices on the bottom of the bun.
Top with 6 meatballs with sauce, 2 tablespoons shredded mozzarella, 1 tablespoon grated Parmesan, and a sprinkle of Italian seasoning.
Bake for 5-6 minutes until the cheese is melted and the bun is toasted. If you are entertaining and serving buffet-style, you can skip baking them.
Video
Notes
Meatballs: Thirty ounces is about 60 meatballs. Bags of frozen meatballs are typically 26-32 ounces and would work for this recipe, even though there are slightly more or less meatballs.
Storage: Store leftovers in an airtight container in the fridge for 3-4 days or in the freezer for 1-2 months.