Preheat the oven to 400°F. Line a baking sheet with parchment paper.

In a large bowl, combine the ground chicken, breadcrumbs, Parmesan cheese, parsley, onion, garlic, egg, oregano, salt, black pepper, and red pepper flakes (if using). Mix until well combined.

Using your hands, form the mixture into meatballs, about 1 to 1.5 inches in diameter, and place them on the prepared baking sheet.

Bake the meatballs in the preheated oven for 20 minutes, or until they are cooked through and golden brown.

While the meatballs are baking, heat the marinara sauce in a saucepan over medium heat.

Once the meatballs are done, remove them from the oven and carefully place them in the saucepan with the marinara sauce. Stir gently to coat the meatballs in the sauce.

Sprinkle the shredded mozzarella cheese over the meatballs and cover the saucepan. Let it sit for a few minutes until the cheese melts.

Serve the meatballs warm, garnished with additional parsley if desired.