Preheat oven to 375°F (190°C).
In a mixing bowl, combine ground beef, breadcrumbs, egg, Parmesan cheese, garlic, salt, and pepper. Mix well.
Form the mixture into meatballs and place them on a baking sheet.
Bake in the preheated oven for 20 minutes, or until cooked through.
While the meatballs are baking, warm the marinara sauce in a saucepan over medium heat.
Once the meatballs are done, place them in the hoagie rolls and top with marinara sauce and shredded mozzarella cheese.
Place the sandwiches back in the oven for 5 minutes, or until the cheese is melted and bubbly.
Serve hot and enjoy!