1. Make meatballs: Preheat oven to 375ºF. Mist a large, rimmed baking sheet with cooking spray. In a large bowl, combine all meatball ingredients and mix gently but thoroughly with your fingers. Using wet hands, shape mixture into 1 1/4-inch balls and place on baking sheet. Bake for 20 minutes, until meatballs are slightly browned on the outside, shaking pan occasionally.
2. In a large bowl, combine all sauce ingredients except scallions, stirring well. Place meatballs in slow cooker, pour sauce over and stir gently. Cover and cook on low for 4 hours. Turn heat to warm setting and serve meatballs directly from slow cooker or, alternatively, transfer meatballs to a serving platter or bowl. Garnish with scallions and serve.