Start by preheating your oven to 350°F (175°C). In a large bowl, mix the ground beef, breadcrumbs, grated Parmesan, egg, garlic, chopped parsley, salt, and pepper. Use your hands or a spoon to mix until everything is well combined but not overmixed.
Roll the meat mixture into 1-inch balls. Place them on a greased or parchment-lined baking sheet. Slide the tray into the oven and bake for about 20 minutes or until the meatballs are cooked through and lightly browned.
While the meatballs are baking, pour the marinara sauce into a saucepan and warm it over medium heat. Once the meatballs are ready, transfer them into the sauce and let them simmer for about 5 minutes. This helps the flavors come together.
Slice open each sub roll. Sprinkle a layer of shredded mozzarella cheese on the bottom half. Spoon the hot meatballs and sauce onto the cheese layer. Top with a little extra cheese if you love it melty and rich.
Place the filled subs on an oven-safe tray and return them to the oven for about 5 minutes, just until the cheese is gooey and irresistible.