Preheat the oven to 375°F (190°C).
In a large bowl, combine ground beef, breadcrumbs, grated Parmesan cheese, chopped parsley, egg, minced garlic, salt, and black pepper. Mix until just combined.
Form the mixture into 16 evenly sized meatballs, about 1 1/2 inches in diameter.
Heat olive oil in a large oven-safe skillet over medium heat. Add meatballs and cook for 5 minutes, turning occasionally until browned on all sides.
Pour marinara sauce over the meatballs in the skillet. Transfer the skillet to the preheated oven and bake for 20 minutes, or until meatballs are cooked through.
While meatballs are baking, slice the hoagie rolls lengthwise without cutting all the way through.
Once meatballs are cooked, remove the skillet from the oven. Using a slotted spoon, place 4 meatballs with sauce into each hoagie roll.
Top the meatballs with 2 ounces of shredded mozzarella cheese per sandwich.
Place the sandwiches on a baking sheet and return to the oven for 5 minutes, or until the cheese is melted and bubbly.
Remove from oven and serve immediately.