Bring 1 litre of water to the boil in a large saucepan while preparing the vegetables. Tip the chopped vegetables into the water and add a pinch of salt. Return the water to the boil, then reduce the heat and cover the pan. Simmer gently for 25 minutes, or until the vegetables are tender.
Remove from the heat, add the garlic and allow to cool slightly. Purée the soup in a blender or food processor until smooth. Return to the pan.
Stir in the sour cream or crème fraîche and reheat gently. Do not allow the soup to boil. Add the snipped chives and season to taste. Garnish with extra chives, if you like.
Chop the celeriac and carrots into roughly equal-sized pieces, so that they take the same amount of time to cook.