Preheat your oven to 375°F (190°C).

In a baking dish, spread a layer of marinara sauce on the bottom.

Add the frozen meatballs in an even layer on top of the sauce.

Pour the remaining marinara sauce over the meatballs.

Sprinkle the shredded mozzarella cheese evenly over the top.

Bake in the preheated oven for 25-30 minutes, or until the meatballs are heated through and the cheese is melted and bubbly.

Slice the sub rolls and arrange them in a single layer in a separate baking dish.

Sprinkle the grated Parmesan cheese over the top of the rolls.

During the last 5 minutes of baking the casserole, place the dish with the sub rolls in the oven to toast.

Serve the meatball sub casserole with the toasted sub rolls.

Enjoy!