Preheat the oven to 375°F (190°C).
In a large bowl, combine ground beef, Italian seasoned breadcrumbs, grated Parmesan cheese, egg, minced garlic, chopped parsley, salt, and black pepper. Mix until just combined.
Shape the mixture into 16 meatballs, about 1 1/2 inches in diameter.
Heat olive oil in a large oven-safe skillet over medium heat. Add meatballs and cook until browned on all sides, about 5-6 minutes.
Pour marinara sauce over the meatballs in the skillet. Transfer the skillet to the preheated oven and bake for 20 minutes, until meatballs are cooked through.
While the meatballs bake, slice the sub rolls lengthwise without cutting all the way through.
Remove the skillet from the oven. Using a slotted spoon, place 4 meatballs with sauce into each sub roll.
Top the meatballs with shredded mozzarella cheese.
Place the assembled 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.