For the meatballs: Mix chicken, bread crumbs, egg, green onion, cornstarch, shredded ginger, soy sauce, salt and pepper together in a large bowl. Cover; refrigerate at least 15 minutes or up to 4 hours.
Heat a large saucepan of water and the crushed ginger to a boil over high heat. Reduce to a simmer.
Using wet hands rinsed under cold water, shape the chicken mixture into 20 meatballs, about 11/2 inches each. Carefully add to the saucepan. Simmer until cooked through, about 6 minutes.
For the teriyaki sauce: Heat the soy sauce, mirin, sugar and rice vinegar to a boil in a medium saucepan over high heat. Boil until thickened and reduced to about 2/3 cup, about 5 minutes.
Remove balls from cooking liquid; drain. Serve over rice, drizzle with the sauce and garnish with green onions.
Nutrition information per serving: