Place the chicken breast fillets on a heatproof plate in a steamer set over gently boiling water. Sprinkle with the mirin or sake and a little seasoning. Cover and steam for 10–12 minutes or until cooked. Set aside to cool.
Mix all the ingredients, except the chilli powder, in a bowl. Drain the cooking juices from the chicken and make up to 1/3 cup (80ml) with water. Add to the dressing. Taste and add more lemon juice, if necessary. Pour into a serving bowl and sprinkle with a pinch of chilli powder.
Cut the cucumber, carrots and red capsicum into fine strips of similar length, about 5cm. Arrange the lettuce leaves at one end of a large platter. Scatter with the shredded basil and mint. Cut the chicken into strips and place on the lettuce. Arrange all the other salad ingredients attractively on the platter. Serve with the dressing in a separate bowl for people to help themselves.