Preheat oven to 400°F. Mix beef, breadcrumbs, Parmesan, egg, garlic, onion, salt, and pepper until just combined.
Form 8 meatballs, pressing a cream cheese cube into center of each and sealing the meat around it.
Sear meatballs in an ovenproof skillet 2 minutes per side, then spoon marinara over them and sprinkle mozzarella.
Bake 12–15 minutes until cooked through and cheese is bubbly.
Spread butter on hoagie rolls, toast under broiler 1–2 minutes, then fill with meatballs and sauce. Sprinkle parsley.