Preheat your oven to 375°F (190°C).
In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic, salt, and pepper.
Mix the ingredients until well combined.
Form the mixture into golf ball-sized meatballs.
Heat a skillet over medium heat and add a little oil.
Brown the meatballs on all sides, about 5-7 minutes.
Add marinara sauce to the skillet and simmer for 15 minutes.
Split the sub rolls and place them on a baking sheet.
Place 3 meatballs in each roll and spoon extra sauce on top.
Sprinkle mozzarella cheese over the meatballs.
Bake in the oven for 10 minutes or until the cheese is melted and bubbly.