01 - Preheat oven to 400°F and position the rack for broiling access.
02 - In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, parsley, oregano, salt, and pepper. Mix until evenly incorporated without overworking the meat. Shape into 12 uniform meatballs.
03 - Heat olive oil in a large skillet over medium heat. Add meatballs and brown on all sides, approximately 5–7 minutes. Remove from the skillet and set aside.
04 - In the same skillet, cook the onion until softened, about 3 minutes. Add minced garlic and stir 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, and reduce heat to low. Simmer for 15 minutes until meatballs are cooked through and the sauce has thickened slightly.
06 - Slice sub rolls lengthwise and optionally brush the cut sides with softened butter. Place under the broiler cut-side up for 1–2 minutes until golden and lightly crisped.
07 - Place 3 meatballs with sauce into each toasted roll. Distribute the remaining sauce evenly. Top generously with shredded mozzarella cheese and arrange subs on a baking sheet.
08 - Broil the assembled subs for 2–3 minutes until the mozzarella is fully melted, bubbly, and lightly browned in spots.
09 - Top with fresh basil leaves if desired and serve immediately while hot.