Shorten the watercress stems a little. Read out the cress, wash and drain. Pluck some cress leaves from the stalks and put them aside for garnishing, chop the rest. Peel and finely chop the onion.
Heat 1 tablespoon of butter in a saucepan. Sauté the onion in it. Dust with flour and sauté briefly until golden brown. Add 400 ml water while stirring. Stir in stock, bring to the boil and simmer for about 5 minutes.
In the meantime cut the bread into small lozenges or cubes. Heat 1 tbsp. butter in a pan and toast bread in it while turning. Season with a little paprika and take out.
Add half the crème fraîche and chopped watercress to the soup and blend briefly with a hand blender. Season with salt, pepper and 1 pinch of sugar. Fill into small bowls, add the rest of the crème fraiche.
Sprinkle with cress leaves and croutons.