In a large mixing bowl, combine the ground beef, ground pork (or turkey), breadcrumbs, grated Parmesan cheese, chopped parsley, minced garlic, egg, salt, and pepper. Mix until well combined.
Take a small amount of the meat mixture (about the size of a golf ball) and flatten it in the palm of your hand. Place a cube of mozzarella cheese in the center, then carefully wrap the meat around the cheese, forming a ball. Repeat with the remaining mixture and cheese.
Heat the olive oil in a large oven-safe skillet over medium heat. Once hot, add the meatballs and cook for about 5-7 minutes, turning occasionally until browned on all sides.
Pour the marinara sauce over the meatballs and bring to a gentle simmer. Cover the skillet and transfer it to the preheated oven. Bake for 15-20 minutes until the meatballs are cooked through and the cheese starts to ooze out.
Remove from the oven and let it cool for a few minutes before serving.
Notes
Serve with crusty bread for dipping and extra parsley for garnish.