For the watercress soup, first peel, halve and finely dice the onion. Melt the butter and sauté the onion until translucent. Add flour and sweat. While stirring, gradually add 250 ml clear soup. Cook gently for 10 minutes with the lid on.
In the meantime, pluck the cress from the stems, rinse and spin dry. Reserve a few sprigs for garnish. Add the remaining cress leaves to the soup and puree the whole thing.
Add 100 g sour cream or crème fraîche and the remaining clear soup and season the soup with salt, pepper and a pinch of sugar.
Divide watercress soup evenly among four plates, serve garnished with the remaining sour cream or crème fraîche and cress.