Dice the onion and heat gently with the butter till golden brown. Deglaze the pot with the white wine, and add the vegetable stock.
Cut the beets into cubes and add them to the soup, season to taste with caraway seeds, salt and pepper, and cook till beets are tender. Puree the soup in a blender and refine with creme fraiche.
For the horseradish yogurt, lightly salt the yogurt and fold in the two teaspoons of prepared or freshly grated horseradish. Form dumplings from the mixture with a spoon and place in the soup. Garnish the finished soup with fresh watercress.