Cut the chicken breasts into long, thin slices. Put 4 teaspoons of the curry paste in a plastic bag with the lemon juice and mix together by squeezing the bag. Add the chicken and toss together.
Half-fill the base of a steamer with water and bring to the boil. Place the chicken in the top of the steamer in a single layer, cover and steam for 5–6 minutes until thoroughly cooked. Test the chicken (see
Meanwhile, mix the remaining curry paste in a bowl with the yogurt.
Cut a thick slice off either side of the mango to reveal the large, flat stone. Trim the flesh away from the stone, then remove the peel and cut the flesh into bite-sized chunks.
Rinse the watercress with cold water and tear it into bite-sized pieces. Add to the yogurt dressing with the cucumber, red onion and mango and toss together gently.
Tear the lettuce into pieces, divide it among 4 plates, spoon the mango mixture on top and complete with the warm chicken strips.