In a small bowl, combine the first seven ingredients. Set mixture aside. Place cornstarch in a large bowl. Add chicken in batches; tossing to coat. In a large skillet, heat 2 tablespoons oil over medium-high heat. Add chicken; cook and stir until no longer pink, 8-10 minutes. Remove and keep warm., Stir-fry onion in remaining oil until tender. Stir soy sauce mixture and add to pan. Bring to a boil; cook and stir 1-2 minutes or until sauce is thickened. Return chicken to pan; heat through. Top with cilantro and green onions.