01 - Preheat the oven to 400°F (200°C).
02 - In a large bowl, combine ground beef, breadcrumbs, Parmesan, egg, garlic, parsley, oregano, salt, and pepper. Mix until just combined, being careful not to overwork the meat.
03 - Shape the mixture into 12 equal meatballs, roughly the size of a golf ball.
04 - Heat a large skillet over medium heat with a splash of oil. Brown the meatballs on all sides, about 5–7 minutes total.
05 - Pour marinara sauce into the skillet, covering the meatballs. Simmer gently for 10–15 minutes, turning occasionally, until the meatballs are cooked through.
06 - Mix softened butter with minced garlic. Slice the sub rolls open and lightly spread with garlic butter. Toast on a baking sheet in the oven for 2–3 minutes until just golden.
07 - Place 3 meatballs and a generous spoonful of sauce into each roll. Top with shredded mozzarella cheese.
08 - Return the assembled subs to the oven and bake for 5–7 minutes, or until the cheese is fully melted and bubbly.
09 - Serve hot, garnished with extra parsley or a sprinkle of Parmesan if desired.