Creating Meatball Sub Casserole is a simple process when you follow these steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Prepare Meatballs: In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
3. Form Meatballs: Shape the mixture into meatballs, about 1 inch in diameter, and place them on a baking sheet.
4. Bake Meatballs: Bake the meatballs in the preheated oven for 20-25 minutes, or until cooked through and browned.
5. Prepare Casserole Dish: While the meatballs are baking, spread a layer of marinara sauce on the bottom of a casserole dish.
6. Layer Ingredients: Once the meatballs are done, add them to the casserole dish on top of the marinara sauce. Pour the remaining marinara sauce over the meatballs.
7. Add Cheese: Sprinkle shredded mozzarella cheese evenly over the top of the casserole.
8. Add Bread: Tear the crusty bread into chunks and place them on top of the cheese.
9. Bake Casserole: Return the casserole to the oven and bake for an additional 15-20 minutes, or until the cheese is melted and bubbly, and the bread is golden brown.
10. Garnish: Remove the casserole from the oven and let it cool for a few minutes. Garnish with fresh parsley, if desired.

Following these steps will help you create an incredible Meatball Sub Casserole that everyone will enjoy.