1. Rinse the leeks and slice them, making sure to use the green tops.
2. Peel and chop the potatoes into 3cm pieces.
3. Separate the tops from the stems of the parsley, then chop up the stems.
4. In a large saucepan, warm the oil and butter, then fry the onion. When soft, add the potatoes, parsley stems and sliced leeks. Cook for about 5 minutes without allowing the vegetables to colour.
5. Add the stock and cook for about 10 minutes, until the potatoes are tender. Allow to cool for a few minutes.
6. Chop the parsley heads and add to the mixture. Blend, then season with salt and black pepper.
7. When the soup has cooled slightly, add the milk, then reheat gently, without allowing it to boil.
8. Serve the smoked chicken on top of each portion and garnish with the chopped parsley.