Preheat oven to 400 degrees. In a large bowl, combine ground chicken, egg, breadcrumbs, salt, onion powder, pepper, and 1/3 cup of green onions. Mix well, roll into 20 round meatballs, about an inch to an inch and a half in diameter. Place on a parchment-lined baking sheet.
Bake meatballs just until the outsides are firmed up, about 5 minutes.
Transfer meatballs into a 4-quart slow cooker. Cover with Buffalo sauce. Place lid on slow cooker and cook on low for 2 hours.
Serve meatballs with blue cheese sauce for dipping, and garnished with a sprinkling of the remaining green onions. Enjoy!
