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 ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, parsley, oregano, salt, and black pepper. Mix gently until evenly incorporated without overworking the meat. Shape into 12 uniform meatballs.
03 - Heat olive oil in a large skillet over medium heat. Add the meatballs in a single layer and brown on all sides, turning carefully, about 5 to 7 minutes total. Transfer the browned meatballs to a plate and set aside.
04 - In the same skillet, add the chopped onion and cook until softened, about 3 minutes. Stir in the minced garlic and cook for 1 additional minute until fragrant.
05 - Pour the marinara sauce and dried basil into the skillet, stirring to combine with the onion mixture. 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.
06 - Slice each sub roll open lengthwise without cutting all the way through. 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.
07 - Place 3 meatballs inside each toasted sub roll and spoon sauce generously over the top. Evenly distribute the shredded mozzarella cheese over each sub.
08 - Place the assembled subs on the baking sheet and broil for 2 to 3 minutes, watching closely, until the mozzarella is fully melted and bubbly with golden spots.
09 - Top each sub with fresh basil leaves if desired and serve immediately while hot.