Cut the tube like body of the squid crosswise into 1/4 inch rings. Wash these rings and the tentacles and put into a pan with 4 tbsps water. Bring to a boil then cover and simmer for about 3 minutes or until the squid turns opaque. Drain and put into a bowl.
Peel and finely slice the shallots.Add to the squid with the lime juice, fish sauce, sugar , chilli powder and coriander leaves. Toss well and check the seasoning.
Just before serving arrange the lettuce leaves on a serving plate. Peel the cucumber and slice into thin rounds. Cut the tomatoes in half vertically.
Put the squid salad on top of the lettuce leaves and surround with the cucumber slices and tomato halves.