Peel and finely chop the onion. Cut off the watercress stems generously. Sort the watercress, wash it and chop it coarsely except for a little bit as an insert. Clean the mushrooms, wash if necessary and cut into slices
Heat the fat in a large pot. Sear the mushrooms in it. Season with salt and pepper and remove. Sauté onion in frying fat. Briefly sweat the flour. Stir in 3/4 l water, stock and 100 g cream, bring to the boil. Simmer covered for about 5 minutes
Whip the rest of the cream until semi-stiff. Add the chopped watercress to the soup and puree finely with a hand blender. Season to taste with salt and pepper. Warm up the mushrooms again. Fold in the cream. Add prawns and the rest of the watercress to the soup.