Break the lettuces into individual leaves. Peel and dice the mango and mix with the finely chopped red onion, chopped mild red chilli, the juice 1 lime and the light olive oil. Mix well and set aside at room temperature, covered with cling film. Just before serving, take a the peeled tiger prawns and cut them in half horizontally. Mix into the mango mixture with a handful chopped coriander. To serve, spoon the mixture on to the Little Gem leaves and arrange on a large platter or plate.