Lightly saute the onion and potato in the butter. Add the watercress, chicken stock and salt and pepper, to taste. Bring to a boil and simmer until the potatoes are cooked through, about 15 minutes. Puree mixture in blender until smooth.
Bring to room temperature and then refrigerate. Serve chilled. Before serving, lightly stir in cream or sour cream and garnish with chopped chives.