Bring the stock to a gentle boil over a moderate heat.

While it is heating, cut the avocadoes lengthwise in half, discard the stone and peel. Cut all but one of the peeled avocado halves into pieces and liquidize at high speed with 4 fl oz of the hot stock; in a few seconds it will have turned into a smooth, thick puree.

Add the cream and blend at high speed for 30 seconds more.

When the stock reaches a gentle boil, reduce the heat to very low and gradually beat in the avocado cream mixture with a balloon whisk or hand-held electric beater, adding a little at a time to blend smoothly. Do not allow to boil.

Remove from the heat as soon as you have added all the avocado mixture, and stir in the sherry and salt and freshly ground white pepper to taste.

Ladle into very hot soup bowls. Cut the remaining avocado half lengthwise into thin slices and place 2 or 3 of these in each bowl of soup.

Serve with hot tortillas.