Creating your Meatball Sub Casserole is straightforward if you follow these simple steps:

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C).
2. Prepare the Meat Mixture: In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic powder, onion powder, salt, and pepper. Mix well until all ingredients are incorporated.
3. Form the Meatballs: Shape the mixture into small meatballs, about 1-inch in diameter. Place them on a baking sheet lined with parchment paper or foil.
4. Bake the Meatballs: Bake the meatballs in the preheated oven for 20-25 minutes or until cooked through and browned.
5. Combine the Casserole Ingredients: In a large mixing bowl, combine the baked meatballs, marinara sauce, and cubed hoagie rolls. Toss to coat everything evenly.
6. Layer the Casserole: In a greased 9×13-inch baking dish, layer half of the meatball mixture. Sprinkle half of the mozzarella cheese on top. Repeat with the remaining mixture and cheese.
7. Bake the Casserole: Transfer the baking dish to the oven and bake for an additional 15-20 minutes, or until the cheese is melted and bubbly.
8. Cool and Serve: Once ready, remove the casserole from the oven and let it sit for a few minutes to cool before serving.

These steps will guide you effortlessly through the process of creating this delightful casserole.