Halve the avocados lengthways. Remove the pits. Rub with lemon juice to prevent the flesh from darkening. Serve cool but not very cold, topping lettuce leaves, surrounded with black olives with a simple vinaigrette or garnished in the following way: With anchovy vinaigrette, prepare a vinaigrette with 4/5 oil and 1/5 vinegar. Season with a spoonful of anchovy paste, garnish the avocado halves with some rolled anchovies. With prawns: use a tablespoonful of prawns for each avocado half, as well as some dices of celery and a spoonful of mayonnaise seasoned with a few drops of avocado and thinned with a few drops of water. With crab meat: same as with the prawns, decorate with some black olives. With tuna: split up the tuna meat, add 1 tablespoonful of tuna and the same quantity of dices of celery to a vinaigrette flavored with very little mustard or to the mayonnaise thinned with water.
