Leeks must be cleaned well, since dirt collects in their leaves as they grow up through the soil.
Cut off the roots and trim away the dried ends of the leaves. Then insert a paring knife, with its sharp edge facing up, through the white of the leek, just below where it starts to turn green. Pull the knife up through the leaves. Give the leek a quarter turn and make a second similar cut.
Spread the leaves of the leeks and wash thoroughly.