Peel and finely chop the onion. Cut the lower stems off the watercress. Wash the watercress. Melt 1 tablespoon of fat in a pot. Fry the onion in it.
Add the watercress, except for something to garnish, and fry briefly. Add stock and 100 g cream, bring to the boil and cook for about 5 minutes. Clean, clean and slice the mushrooms. Heat 1 tablespoon of fat, fry the mushrooms in it. Beat 100 g cream until semi-stiff. Puree watercress in the broth. Season to taste with salt and pepper. Fold in the cream. Arrange soup in bowls.
Beat 100 g cream until semi-stiff. Puree watercress in the broth. Season to taste with salt and pepper. Fold in the cream. Arrange soup in bowls. Arrange crab meat, mushroom slices and some watercress on top.