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 meatballs reach 160°F and cheese bubbles.
Spread butter on hoagie rolls, toast under broiler 1–2 minutes, then fill with meatballs and sauce. Sprinkle parsley.