Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or lightly grease it with non-stick spray.
In a large mixing bowl, combine ground turkey, breadcrumbs, grated Parmesan cheese, chopped parsley, diced onion, minced garlic, egg, salt, black pepper, Italian seasoning, and red pepper flakes (if using).
Gently mix all the ingredients until well combined. Be careful not to overmix.
Form the mixture into meatballs about 1-1.5 inches in diameter and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for 20-25 minutes, or until cooked through and reaching an internal temperature of 165°F (75°C).
Optionally, pan-sear the meatballs in a skillet over medium heat for 3-4 minutes on each side for added browning before transferring them to the oven.
Remove the meatballs from the oven and let them rest for a few minutes before serving. Serve with marinara sauce on top or on the side, if desired.