Cook the peeled prawns in boiling water with juice from half a lemon for 30 seconds. Remove them from the water, cool them down in iced water and strain them once cool. Set aside.
Slice the avocados down the middle, remove the pit and hollow out with a scoop or a dessert spoon.
Peel an orange, remove all the whitish strands from the skin and cut it into small cubes.
Cut the lettuce leaves in julienne type pieces (thin strips) and mix with the diced flesh of the avocado, the boiled prawns and the orange cubes and set aside.
In a cup or bowl, make a cocktail sauce by mixing a bit of ketchup, mayonnaise and juice from half a lemon.
Put the lettuce, avocado, prawn and orange mix inside the avocados and pour the sauce over it.
Decorate the avocados with the peeled prawns.
Dust some minced parsley over them and serve chilled.
