01 - Preheat the oven to 400°F (200°C).
02 - In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, parsley, oregano, salt, and pepper. Mix gently until just combined, being careful not to overwork the meat.
03 - Shape the mixture into 12 equal-sized 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, approximately 5–7 minutes total.
05 - Pour the marinara sauce into the skillet, submerging the meatballs. 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 inside with garlic butter. Toast on a baking sheet in the oven for 2–3 minutes until golden.
07 - Place 3 meatballs and a generous spoonful of sauce into each toasted roll. Top evenly 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 immediately while hot. Garnish with extra parsley or a sprinkle of Parmesan if desired.