Combine the frozen meatballs, marinara, beef broth, Italian seasoning and garlic powder in a 6-quart crockpot and stir to coat.

Cover and cook on low for 6 hours or high for 3 hours, until the meatballs reach 165 degrees and the sauce is bubbling and thick enough to coat the back of a spoon.

Split and lightly toast the hoagie rolls under the broiler for 1 to 2 minutes, watching closely, until the edges turn golden.

Fill each roll with 4 to 5 meatballs and a generous spoonful of sauce.

Top each sub with mozzarella and a pinch of parmesan.

Return filled subs to a sheet pan and broil for 1 to 2 minutes, until the cheese melts and bubbles at the edges.

Serve immediately, straight from the crockpot for refills.