Carefully trim the stringy roots from the leeks, leaving the tip of the bulb intact. Trim the green tops. Then split each leek down the middle almost to the root end. Wash them well (leeks are very sandy) in a large bowl of cold water, changing the water until clear.
Bring a large saucepan of lightly salted water to a boil. Add the leeks and simmer gently until tender and the bulb end o