Preheat oven to 400°F (200°C).
In a mixing bowl, combine ground beef, breadcrumbs, egg, Parmesan cheese, garlic, salt, and pepper. Mix until well combined.
Form the mixture into meatballs and place them on a baking sheet.
Bake in the preheated oven for 15-20 minutes, or until fully cooked.
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, top with marinara sauce and shredded mozzarella cheese.
Place the sandwiches on a baking sheet and bake for an additional 5 minutes, or until the cheese is melted and bubbly.
Serve hot and enjoy!