Preheat the oven to 375°F (190°C).
In a large bowl, combine ground beef, Italian-style breadcrumbs, grated Parmesan cheese, egg, minced garlic, chopped parsley, salt, and black pepper. Mix well until all ingredients are evenly incorporated.
Shape the mixture into 16 meatballs, about 1 inch in diameter.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the meatballs in batches and cook until browned on all sides, about 5 minutes. Transfer browned meatballs to a plate.
Pour marinara sauce into the skillet and bring to a simmer. Return the meatballs to the skillet, cover, and simmer for 15 minutes until cooked through.
While the meatballs are simmering, brush the sub rolls with the remaining 1 tablespoon of olive oil and place them on a baking sheet. Toast in the oven for 5 minutes until lightly crisp.
Remove the rolls from the oven and spoon 4 meatballs with sauce into each roll.
Sprinkle shredded mozzarella cheese evenly over the meatballs in each sub.
Place the assembled subs back in the oven and bake for 5-7 minutes, or until the cheese is melted and bubbly.
Remove from the oven and serve immediately.