Pick off the leaves of cress and chop fine. Cut onions in thin slices. Cook watercress and onions in butter five minutes (without browning), add flour and salt, stir until smooth, then pour milk on gradually, stirring constantly. Cook over hot water twenty minutes. Add beef extract, stir until dissolved; season with Worcestershire sauce and a few grains cayenne. Strain into hot soup tureen, add whipped cream and sprinkle with finely chopped parsley.