Preheat your oven to 375°F (190°C).
In a mixing bowl, combine ground beef, breadcrumbs, egg, salt, and black pepper. Mix well.
Form the mixture into meatballs and place them on a baking sheet.
Bake the meatballs in the preheated oven for 20 minutes, or until cooked through.
While the meatballs are baking, heat the marinara sauce in a saucepan over medium heat.
Once the meatballs are done, add them to the marinara sauce and simmer for 10 minutes.
Place the hoagie rolls on a baking sheet and toast them in the oven for 5 minutes.
Assemble the sandwiches by placing meatballs and sauce on the toasted hoagie rolls. Top with shredded mozzarella cheese.
Serve hot and enjoy!