Make meatballs: In a large bowl, combine ground beef, eggs, bread crumbs, 1/4 cup parsley, 3 cloves garlic, salt, and pepper. Shape into golf ball-sized meatballs.
Cook meatballs: Heat a cast iron pan with oil over medium-high heat. Cook meatballs until browned on all sides. Transfer to a plate and set aside.
Preheat oven: Preheat oven to 350°F (180°C).
Prepare rolls: Place soft rolls in a baking sheet. Cut the top and press the inside down to create boats.
Make garlic butter: Mix melted butter, 3 cloves garlic, and 2 tablespoons parsley.
Toast rolls: Brush rolls with garlic butter and bake for 5 minutes, or until toasted.
Assemble boats: Place mozzarella slices in rolls. Add a spoonful of marinara sauce.
Add meatballs: Place 3 meatballs in each boat and top with more marinara sauce.
Bake and serve: Bake for 25 minutes, or until cheese is melted. Top with Parmesan cheese and serve.