Cook the soup mix according to the directions on the packet and leave to cool. Peel and slice the avocado, keeping back some of the slices for decoration.
In a food processor, combine the cold soup mix, avocado, onion and seasoning and blend on a low speed for 1 minute, then turn up to high and purée the mixture until it becomes thick and creamy. Pour into a clean bowl and chill in the fridge.
Ten minutes prior to serving the soup, stir in the cream and the reserved avocado slices. Taste and adjust the seasoning if necessary. Garnish with chopped chives and serve the same day.
To ensure that the avocado slices for the garnish do not brown, dip in lemon juice quickly after slicing.