In a large bowl, combine ground beef, breadcrumbs, Parmesan, egg, garlic, Italian seasoning, salt, and pepper. Mix until just combined, then form into 12 meatballs.
Heat olive oil in a large skillet over medium heat. Brown the meatballs on all sides, about 8 minutes total.
Pour marinara sauce over the meatballs, reduce heat to low, cover, and simmer for 15 minutes until meatballs are cooked through.
While meatballs simmer, preheat your broiler. Split the sub rolls and lightly toast them under the broiler for 1-2 minutes.
Place 3 meatballs in each toasted roll, spoon extra sauce over the top, and generously sprinkle with mozzarella cheese.
Return sandwiches to the broiler for 2-3 minutes until cheese is melted and bubbly. Garnish with fresh basil if desired and serve immediately.