Preheat the oven to 400°F (200°C).
In a large bowl, combine ground beef, grated Parmesan cheese, breadcrumbs, egg, minced garlic (2 cloves), dried Italian seasoning, salt, and black pepper. Mix until just combined.
Form the mixture into 16 meatballs, about 1 inch in diameter.
Place the meatballs on a baking sheet lined with parchment paper and bake for 15 minutes.
While the meatballs bake, warm the marinara sauce in a medium saucepan over low heat.
After 15 minutes, remove the meatballs from the oven and add them to the marinara sauce. Simmer for 5 minutes to heat through and absorb the sauce flavors.
Meanwhile, slice the sub rolls lengthwise without cutting all the way through, creating a pocket.
In a small bowl, mix the softened butter with the minced garlic (3 cloves).
Spread the garlic butter mixture evenly on the cut sides of each sub roll.
Place the rolls on a baking sheet and toast in the oven for 5 minutes until golden and crispy.
Remove the toasted rolls from the oven. Spoon 4 meatballs with sauce into each roll.
Top each sub with 1/4 cup shredded mozzarella cheese.
Return the assembled subs to the oven and bake for an additional 5 minutes or until the cheese is melted and bubbly.
Remove from the oven and sprinkle with chopped fresh parsley before serving.