Using gloves, peel and coarsely chop the beetroot. Peel and chop the potatoes. Place in a pot with the vegetable stock and cook until tender, then chill.
Peel and roughly chop eschallots, chop cucumber and place in a blender with the cooled beetroot mix and kefir. Blend until smooth, then top with enough cold water to make 1.2L. Blend again to mix and season to taste with salt, pepper and as much lemon juice as you like.
Pour into bowls, top with a dollop of crème fraiche, chopped chives and finely grate horseradish over the top.
If you can’t find fresh horseradish, use “eksal” or “krakus” brand preserved horseradish, available in the international or kosher section of your supermarket, or at any Polish or Russian deli. Blend the preserved horseradish with the ingredients in step 2.