Blanche the watercress in a large saucepan of salted, boiling water for 30 seconds. Drain and refresh in iced water. Cook the green beans in a large saucepan of salted, boiling water for 3-4 minutes until tender. Drain and transfer to a food processor.
Add the blanched watercress, parsley leaves, butter and a little seasoning. Puree until smooth. Spoon into serving bowls and garnish with the raw beans, parsley leaves and sprig of watercress on the side.