Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper and lightly grease it with cooking spray or olive oil.

In a large mixing bowl, combine the ground chicken, breadcrumbs, Parmesan cheese, parsley, onion, garlic, egg, salt, black pepper, Italian seasoning, and red pepper flakes (if using). Mix until just combined; be careful not to overmix.

Using your hands, form the mixture into meatballs about 1 to 1.5 inches in diameter. Place the meatballs on the prepared baking sheet, leaving space between each one.

Bake the meatballs in the preheated oven for 20-25 minutes, or until they are cooked through and golden brown on the outside.

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

Once the meatballs are done, remove them from the oven and let them cool for a few minutes. Serve the meatballs with the marinara sauce on top or on the side for dipping.