In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, grated Parmesan cheese, chopped parsley, minced garlic, beaten egg, and salt and pepper. Mix gently until just combined.

Divide the mixture into portions and flatten each into a patty. Place a teaspoon of mozzarella cheese in the center, fold and seal the meat around the cheese, then roll to form meatballs.

Heat olive oil in a large skillet over medium heat. Add meatballs in batches, cook for about 5-7 minutes until golden brown and cooked through.

Preheat oven to 375°F (190°C). Transfer meatballs to a baking dish, pour marinara sauce over, sprinkle more Parmesan on top, and bake for 15-20 minutes.

Remove from oven, cool slightly, and garnish with parsley or Parmesan before serving.