1.Place buk choy and choy sum on base of a large bamboo steamer. Cut each chicken breast diagonally crossways into three pieces; place on vegetables.
2.Cut ginger lengthways into thin strips, cut into matchstick-sized pieces. Sprinkle ginger, peppercorns, rind and oil over chicken. Season. Steam chicken, covered, over a wok or large frying pan of simmering water 10 minutes or until cooked through.
3.To make soy sesame dressing; whisk ingredients in a small jug until combined.
4.Serve chicken and vegetables, topped with green onion and drizzled with dressing.