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, about 1 inch in diameter.
Heat a skillet over medium heat and cook the meatballs until browned on all sides, about 5-7 minutes.
Transfer the meatballs to a baking sheet and bake in the preheated oven for 10 minutes, or until cooked through.
Warm the marinara sauce in a saucepan over medium heat.
Place meatballs in hoagie rolls, spoon marinara sauce over the meatballs, and top with shredded mozzarella cheese.
Place the sandwiches on a baking sheet and bake for 5 minutes, or until the cheese is melted and bubbly.
Serve hot and enjoy!