Preheat oven to 400º. In a large bowl, combine ground chicken, bread crumbs, Parmesan, parsley, half the garlic, and egg and season with salt and pepper. Mix until fully combined, then form into meatballs.
In a large oven proof skillet, heat 1 tablespoon oil over medium heat. Add meatballs and brown each side, 5 minutes. Transfer to a plate.
Add remaining 2 tablespoons oil to skillet. Add remaining half garlic and cook until fragrant, 1 minute, then stir in crushed tomatoes and red pepper flakes.
Bring to a simmer, then return meatballs to skillet. Top with mozzarella and bake until cheese is melted and chicken is no longer pink, 10 minutes more.
Garnish with parsley before serving.
