Combine sausage, bread crumbs, egg and 2 tablespoons green onion; mix lightly but thoroughly. Shape into ¾-in. balls.
In a large saucepan, bring broth and soy sauce to a boil. Carefully drop meatballs into soup. Stir in bok choy, celery and noodles; return to a boil. Cook, uncovered, until meatballs are cooked through and noodles are tender, 10-12 minutes. Stir in basil and remaining green onions; remove from heat.
