In a large pan, melt the butter and gently fry the onion and garlic. Add the leek and celery and fry for about 2 minutes over medium heat.
Add the stock and bring to the boil. Reduce the heat and add the watercress and let it simmer for about 5 minutes.
Puree the soup with a hand blender or in a blender until smooth. Season with pepper and salt if necessary.
Add some cream and serve with chopped chives.