Set a steamer over a pan of simmering water and steam the king prawns for 2–3 minutes until cooked and pink. Set aside and leave to cool.
Make the dressing by mixing together all the ingredients in a small bowl.
Toss together the bean sprouts, mangetout, water chestnuts, lettuce and radishes and scatter over the prawns and sesame seeds. Drizzle over the dressing and serve immediately.