For the beetroot soup, first rinse and peel the potatoes and cut into cubes. Chop the onions. Heat oil in a saucepan, sauté onions and potatoes over medium heat for 5 minutes. Peel beet and cut into small pieces. Add to onions and potatoes.
Pour in clear soup and bring to a boil, cook over medium heat for 30 to 40 minutes. Add whipped cream and purée finely. Season with salt and pepper. Strain through a sieve if necessary.
Divide beetroot soup evenly into four baking bowls and bring to table, garnished with 1 teaspoon of horseradish.