Take a large pan and cook the onion in the butter until soft but not browned.
Add the potatoes and stock and cook until the potatoes are soft (approx 15-20 minutes).
When potatoes are almost cooked, add the watercress (try to allow for no longer than 5 minutes cooking time for the watercress).
Liquidise and add the cream. Use a little extra milk if the soup seems too thick.
Season with salt and black pepper and serve hot.
Calories: 114: Fat: 8.3g Saturated Fat: 4.9g Carbohydrate: 14g Protein: 4.2g Fibre: 1.8g Salt: 0.95g