Peel and finely chop onion. Melt half butter in a large pan, add onion and fry until soft. Chop mushrooms finely. Add half to onion in pan and cook until soft. Pour stock into saucepan and bring to boil. Wash and trim watercress. Reserve a few leaves for garnish. Plunge watercress into boiling stock and leave for about 30 seconds, until limp and emerald green in colour. Remove pan from heat. Puree soup immediately in a blender or food processor to set a bright green colour. Rinse pan. Return soup to pan, passing it through a sieve. Melt remaining butter in a small pan and fry remaining chopped mushrooms. Add Madeira to pan and reduce to evaporate liquid. Add cream and bring to boil. Reduce again to thicken and slightly caramelise cream, giving a nuttie flavour. Stir caramelised cream into watercress puree and heat gently. Season to taste with salt and pepper. Garnish with reserved watercress leaves before serving.