Heat the butter in a pan, add the onion, fennel and leeks and fry gently until softened slightly. Add the garlic and fry for one minute.
Add the stock and bring to the boil. Reduce the heat and simmer for 6-8 minutes, or until the vegetables are tender. Remove from the heat and allow to cool slightly.
Pour the soup into a blender. Add the cream, half of the chives and the lemon juice. Season, to taste, with salt and freshly ground black pepper and blend until smooth.
To serve, pour the soup into a bowl and garnish with the remaining chives.