To make the dressing, mix all of the ingredients together in a small pot and bring to the boil. Remove from the heat and leave to cool.
Remove the chicken from the bones and slice thickly across the grain. Put the chicken, onion and tomato in a bowl, drizzle with the dressing and gently toss together. Put the lettuce on a serving plate and arrange the chicken salad on top. Sprinkle with peanuts to serve.