Put the soy sauce, sake and sugar in a large, deep frying pan and stir to dissolve the sugar. Bring to the boil and simmer for 3–4 minutes. Add the pork and simmer gently for 7–8 minutes, or until the pork is just cooked through, turning occasionally. Remove from the heat and leave to rest.
Place the pak choi in a steamer basket and lower into a shallow saucepan of boiling water so that the pak choi is not quite touching the water. Cover and steam for 3–4 minutes or until tender. Alternatively, use a bamboo or electric steamer.
Arrange the pak choi in serving dishes with the pork and its cooking liquid and steamed Thai jasmine rice. Sprinkle over sesame seeds and serve immediately.