Prawn and two-fruit cocktail

To make the dressing: Whisk all the ingredients together in a small bowl or jug. To assemble the salad: Peel and segment the grapefruit over a plate to catch the juices. Cut the avocado in half and take out the stone. Slice avocado fairly thinly, then with a sharp knife, slice the flesh away from the skin, adding the slices to the grapefruit juice on the plate as you go, to prevent them browning and to give them flavour. Arrange the whole or torn lettuce leaves in 4 glass dishes. Add the grapefruit segments, avocado and prawns, and drizzle with dressing. Season.