Cut each mozzarella string cheese stick into 4 equal pieces, giving you 12 pieces total. Set aside.

Roughly chop the pepperoni so it gets evenly distributed throughout the meatballs. Set aside.

In a large bowl, combine the ground meat, chopped pepperoni, egg, breadcrumbs, Parmesan, Italian seasoning, garlic powder, onion powder, salt, and pepper. Mix gently until just combined.

Divide the mixture into 12 equal portions. Flatten each portion in your hand, place one piece of mozzarella in the center, and carefully form the meat around the cheese, completely sealing it inside. Roll into a ball.

In a shallow bowl, combine the panko, Parmesan, Italian seasoning, garlic powder, salt, pepper, and red pepper flakes if using.

Roll each meatball in the panko mixture, gently pressing the crumbs onto the outside so they stick.

Place the meatballs on a baking sheet lined with parchment paper and lightly spray the tops with olive oil spray. Bake at 400°F for 20-25 minutes, or until the turkey reaches 165°F in the center.

Warm the marinara sauce and serve the pizza meatballs alongside for dipping. Garnish with chopped fresh parsley or basil if desired.