Follow these simple steps to create your Meatball Sub Casserole:

1. Preheat the Oven: Begin by preheating your oven to 375°F (190°C).
2. Prepare the Meatballs: In a mixing bowl, combine the ground beef (or turkey), Italian bread crumbs, grated Parmesan cheese, egg, minced garlic, Italian seasoning, salt, and pepper. Mix until well combined.
3. Form Meatballs: Shape the mixture into small meatballs, about 1 to 1.5 inches in diameter. Place them on a baking sheet lined with parchment paper.
4. Bake Meatballs: Bake the meatballs for approximately 20 minutes or until they are browned and cooked through.
5. Add Sauce: While the meatballs are baking, heat the marinara sauce in a saucepan over low heat until warmed through.
6. Combine in Casserole Dish: In a casserole dish, add the baked meatballs and pour the warmed marinara sauce over them. Mix gently to combine.
7. Add Hoagie Roll Pieces: Arrange pieces of the hoagie roll on top of the meatball mixture to recreate the sub effect.
8. Top with Cheese: Sprinkle shredded mozzarella cheese evenly over the top of the casserole.
9. Bake Casserole: Return the casserole to the oven and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden.
10. Garnish: Once out of the oven, let it cool slightly, then garnish with freshly chopped basil or parsley, if desired.

Enjoy every delightful bite of your Meatball Sub Casserole!