Put the prawns in a glass or ceramic bowl. Add the sesame oil, soy sauce, honey, ginger, garlic and lemon juice. Season to taste with salt and pepper and mix well, then cover and leave to marinate in a cool place for 5–10 minutes.
Cut the heads of pak choi in half lengthways, then blanch in a large saucepan of boiling water for 40–50 seconds. Drain well, cover and keep warm.
Heat the vegetable oil in a wok or large frying pan. Add the prawns and marinade and stir-fry over a high heat for 2 minutes until thoroughly hot.
Divide the pak choi between 4 serving plates, then top with the prawns and any juices from the pan. Serve immediately.