Heat the oil in a large saucepan, add the onion, celery and thyme and cook gently for 3–4 minutes. Add the beetroot and vinegar and cook for 2 minutes.
Pour over the stock, cover and simmer for about 25 minutes until the beetroot is tender. Season to taste with salt and pepper and stir in the horseradish sauce.
Blend the soup with a stick blender until smooth, or transfer to a food processor or blender, in batches, to blend. Reheat gently if necessary.
Mix the soured cream or crème fraîche with the remaining horseradish sauce. Spoon on top of the soup and garnish with chopped chives. Serve with crusty bread.