In a large mixing bowl, combine the ground chicken, breadcrumbs, chopped green onions, minced garlic, grated ginger, soy sauce, sesame oil, honey, egg, salt, black pepper, and red pepper flakes (if using). Mix until well combined.
Using your hands, form the mixture into small meatballs (about 1-inch in diameter). You should get around 20-24 meatballs.
(Optional) For a slight crust, bake the meatballs on a baking sheet at 400°F (200°C) for 15 minutes before adding them to the crockpot.
Place the meatballs in the crockpot. Pour the teriyaki sauce over the meatballs, ensuring they are well-coated.
Cover the crockpot and cook on low for 6 hours, or on high for 3 hours, until the meatballs are cooked through and tender.
Once cooked, use tongs or a slotted spoon to serve the meatballs with the sauce.
Garnish with sesame seeds and additional chopped green onions if desired. Serve over rice or noodles.