Heat half the butter in a pan, add the onion and leek and cook over a low heat for 4-5 minutes until softened. Add the potatoes and cook for a further 1 minute. Pour over the stock, reduce the heat and simmer for 20 minutes or until the potatoes are soft. Transfer to a food processor, add the milk and remaining butter and blend until smooth in texture.
Bring a pan of water to the boil,