Preheat your oven to 400°F. Line a baking sheet with parchment paper.
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 well combined.
Using your hands, form the mixture into meatballs about 1 inch in diameter and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for 20-25 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, add them to the marinara sauce and let them simmer for about 5 minutes to absorb the flavors.
Serve the meatballs with pasta, on a sub roll, or on their own with extra sauce on the side.