In a saucepan over a medium heat, melt the butter and add the onion and garlic. Cook and stir for 3 to 4 minutes until soft. Stone 3 1/2 of the avocados and chop. Reserve the remaining 1/2 avocado and slice thinly for a garnish; set aside. To the pan, add the chopped avocados, stock, single cream, lemon juice, chives and season well. Allow to cook gently for 7 to 8 minutes.
Purée the soup using a hand-held liquidiser until thick and creamy. When it is time to serve, garnish with a dollop of crème fraiche, a thin sliver of avocado, some parsley and a sprinkle of pine nuts. Serve hot.