watercress soup

a delicious and nutritious soup, which is great served with crusty bread or biscuits like these:
http://menuinseason.blogspot.com/2013/02/sour-cream-biscuits.html

serves 4

1 Tbsp. oil
1 Tbsp. butter
1 onion, finely chopped
1 potato, peeled and diced
6 ounces watercress
14 oz. (1 2/3 cup) vegetable stock
14 oz. (1 2/3 cup) milk
lemon juice, to taste
salt and pepper
sour cream to serve

1. Heat oil and butter in a large, heavy pan and cook the onion over gentle heat for about 5 minutes, until soft but not browned. Add potato, cook gently 2-3 minutes and then cover and cook over gentle heat for 5 minutes, stirring occasionally.
2. Meanwhile, strip the watercress leaves from the stalks and coarsely chop the stalks.
3. Add the stock and milk to the pan, stir in the chopped watercress stalks and season to taste with salt and pepper. Bring to a boil, lower the heat and summer gently, partially covered, for 10-12 minutes, until the potatoes are tender. Add all but a few of the watercress leaves and simmer for 2 minutes more.
4. Process the soup in a blender, then pour into a clean pan and heat gently with the reserved leaves. Add a little lemon juice and adjust seasoning.
5. Garnish with sour cream and enjoy!