Boil the beetroot whole in salted water for 15 minutes. Drain and refresh in cold water. Peel and cut into 2.5cm/1in chunks.
In a frying pan, gently sweat the onion, leek and celery in the butter until softened.
Add the beetroot, stock and bay leaf. Bring to the boil, skim the surface, reduce the heat and simmer for 45 minutes.
Discard the bay leaf, and purée the soup in a blender or food processor. Return to a clean pan, season and bring back to the simmer. Remove from the heat, stir in the sour cream and serve.