Put lamb's lettuce and watercress from basic recipe in a large bowl, add prawns.
Peel the grapefruit with a sharp knife, slice off the top and bottom, place on a chopping board and trim away the skin and pith by cutting from top to base all the way around the fruit. Cut each of the segments away from core, add to the leaves and squeeze the juice from the fruit into the leaves.
Add classic vinaigrette dressing and toss. Divide between two plates. Cut the avocado in half and remove stone, peel and slice onto the salad. Sprinkle the mixed seeds over.
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