1. Pinch heads from bodies of prawns.  Pinch tail section away, then peel shell away from center part of body.  If you want to keep tails intact, leave tail and next section of shell in place, then pull away centre shell, as shown.

2. Remove back veins by pulling veins from head end towards tail end.

3. Using sharp knife, cut top and bottom from grapefruit, cut away skin and white pith.

4. Cut between membranes of each grapefruit segment.  Line serving dishes with lettuce leaves, place prawns, grapefruit and avocado into dishes, spoon cocktail sauce over prawn mixture.  Garnish with prawns (with tails intact), lemon and a spring of dill or parsley, if desired.