01 - Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or foil.
02 - In a large mixing bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, parsley, oregano, salt, and pepper. Gently mix until just combined, being careful not to overwork the meat.
03 - Roll the mixture into 12 evenly sized meatballs, approximately golf ball size.
04 - Heat a large skillet over medium heat with a light drizzle of oil. Brown the meatballs on all sides, approximately 5–7 minutes total.
05 - Pour marinara sauce over the browned meatballs in the skillet. Reduce heat and simmer gently for 10–15 minutes, turning occasionally, until the meatballs are cooked through.
06 - Combine softened butter with minced garlic. Slice the sub rolls open and lightly spread the cut sides with garlic butter. Toast on a baking sheet in the oven for 2–3 minutes until lightly golden.
07 - Place 3 meatballs with sauce into each toasted roll. Generously top with shredded mozzarella cheese.
08 - Return the assembled subs to the oven and bake for 5–7 minutes, or until the mozzarella is fully melted and bubbly.
09 - Serve immediately while hot. Garnish with additional fresh parsley or grated Parmesan if desired.