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 (about 15 to 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 everything together 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
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