Dry the leek on absorbent paper. Heat 1cm oil in a frying pan over medium-high heat. Cook leek, in batches, until golden and crispy. Drain on absorbent paper and set aside.
Place the lettuce on serving plates and top with avocado and prawns.
To make the dressing, combine the mayonnaise, lime juice, water and pepper. Spoon over the prawns and avocado and top with the crispy leek. Serves 8.