Preheat your oven to 375°F (190°C).
In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, parsley, egg, minced garlic, salt, and black pepper.
Form the mixture into even-sized meatballs, about 1.5 inches in diameter, and place them on a baking sheet lined with parchment paper.
Bake the meatballs for about 20-25 minutes or until cooked through and nicely browned.
While your meatballs are baking, warm the marinara sauce in a saucepan over medium heat.
Take the baked meatballs out of the oven and pour the warmed marinara sauce over them, gently coating each one.
Slice the hoagie rolls down the center, being careful not to cut all the way through, and place them on a baking sheet.
Brush the inside of the rolls with melted garlic butter.
Stuff each roll with a generous amount of meatballs, followed by a ladle of marinara sauce.
Sprinkle shredded mozzarella on top of each stuffed roll and place the tray in the oven.
Bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let cool for a few minutes before serving.
