01 - Preheat oven to 400°F.
02 - In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, parsley, oregano, salt, and pepper. Mix until evenly incorporated, then shape into 12 equal-sized meatballs.
03 - Heat olive oil in a large skillet over medium heat. Add the meatballs and brown on all sides, about 5–7 minutes. Remove and set aside.
04 - In the same skillet, cook the onion until softened, about 3 minutes. Add the garlic and cook for 1 additional minute. Pour in the marinara sauce and dried basil, then bring to a simmer.
05 - Return the browned meatballs to the skillet. Cover, reduce heat to low, and simmer for 15 minutes until the meatballs are cooked through.
06 - Slice the sub rolls and optionally brush the insides with softened butter. Toast the cut sides under the broiler for 1–2 minutes until golden.
07 - Place 3 meatballs and sauce into each toasted roll. Top generously with shredded mozzarella cheese and arrange the subs on a baking sheet.
08 - Broil the assembled subs for 2–3 minutes until the cheese is melted and bubbly.
09 - Garnish with fresh basil leaves if desired and serve immediately while hot.