Halve the avocados lengthways. Remove the core and peel off the skin. Drizzle the lemon juice over the avocado halves and cut 3 halves into pieces. Cover and set aside the other half of the avocado.
Heat the broth. Puree the avocado pieces with the hot stock and the crème fraiche. Put the soup in a saucepan and bring to the boil while beating with the whisk. Stir in the sherry and remove the soup from the stove.
Cut the remaining half of the avocado into small cubes and mix with the soup. Serve immediately.