1 tablespoon cornflour, mixed to a paste with 2 tablespoons water
Heat the sesame oil in a large, heavy-based frying pan, add the chicken breasts and fry, turning once, for 5 minutes or until browned. Add the chilli to the pan with the lemon rind and juice. Cover and simmer for 15 minutes or until the chicken is cooked.
Meanwhile, steam or lightly cook the pak choi in a little lightly salted boiling water until just tender.
Remove the chicken from the pan and keep warm. Stir the cornflour paste into the pan juices and bring to the boil, stirring until thickened and adding a little water if the sauce is too thick. Serve the chicken with the pak choi and the lemon sauce poured over the top.