Wash the watercress and remove the stalks. Reserve 24 leaves for decoration. Melt 40 g (1½ oz) butter in a medium-sized sauté pan, and soften the chopped shallot. Add the watercress and season. Cover the pan and stew gently over a low heat for 20 minutes. When the watercress has melted down spread it out on a board and cut it into shreds with a knife. In a small saucepan