Place the frozen meatballs, Mariana sauce, mini pepperoni, and half the shredded Parmesan cheese in a crock pot. Add the minced garlic, Italian seasoning, black pepper and salt. With a large spoon toss the ingredients.
Cook on high for 4 hours or low for 6-8 hours.
30 minutes before you are ready to serve the subs sprinkle the remaining shredded mozzarella cheese and Parmesan cheese over the top of the meatballs, replace lid and continue to cook until cheese is melted.
Serve on toasted sub rolls or slider buns.
Sprinkle with remaining Parmesan cheese before serving.