MELT butter in a large soup pot. (We prefer salted butter, but use whichever you prefer and adjust salt as desired at the end.
ADD the leeks and potatoes, stirring to coat well with the butter.
COVER and cook gently for 5 minutes.
ADD soup stock and simmer for about 15 minutes, or until the vegetables are tender.
REMOVE from heat and allow to cool for 5 minutes or so.
ADD watercress, chopped, to keep stems from winding around blender blades, (or arugula or other substitute); (reserve a few tablespoons of leaves for garnish).
BLEND with a hand blender or pitcher blender to liquidize. If you prefer a partial blending so you have some pieces to chew, then use the hand blender or set aside some of the soup before you blend it.
SEASON to taste.
FOR HOT SOUP - stir in the dairy, top with garnish and serve.
FOR CHILLED SOUP - stir in the dairy and chill
GARNISH with chives, watercress or parsley, if desired.