01 - Preheat oven to 400°F. Position the oven rack so the broiler can reach the tops of the subs later.
02 - In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, parsley, oregano, salt, and black pepper. Mix gently until evenly incorporated, being careful not to overwork the meat. Shape the mixture into 12 uniform meatballs.
03 - Heat olive oil in a large skillet over medium heat. Add the meatballs and brown on all sides for 5 to 7 minutes, turning occasionally for even color. Remove the meatballs from the skillet and set aside on a plate.
04 - In the same skillet, add the chopped onion and cook until softened, about 3 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
05 - Pour the marinara sauce into the skillet and stir in the dried basil. Bring to a gentle simmer, then return the browned meatballs to the pan. Cover, reduce heat to low, and simmer for 15 minutes until the meatballs are cooked through to an internal temperature of 160°F.
06 - Slice each sub roll open lengthwise, leaving one side attached as a hinge. Optionally brush the cut sides with softened butter. Place rolls cut-side up on a baking sheet and toast under the broiler for 1 to 2 minutes until golden brown.
07 - Place 3 meatballs inside each toasted sub roll and spoon sauce generously over the top. Evenly distribute the shredded mozzarella cheese over each sandwich.
08 - Return the assembled subs to the baking sheet and broil for 2 to 3 minutes, watching closely, until the mozzarella is fully melted and bubbling with golden spots.
09 - Remove from the oven, garnish with fresh basil leaves if desired, and serve immediately while hot.