Peel and chop the onion. Roughly dice the beetroot. Peel, wash and chop the potatoes. Heat the oil in a pot. Brown everything in it. 3⁄4 l Pour on water and beetroot juice, bring to the boil and simmer for about 15 minutes.
Rinse and drain the sprouts. Wash the chives, shake dry and cut into small rolls.
Puree the soup very finely with a hand blender. Stir in horseradish and buttermilk and heat up, but do not boil any more. Season to taste with salt and pepper. Serve with chives and sprouts.