Leeks are grown in sandy soil that is piled up around them to keep the stem white, so it's important to wash them well. Firstly, rinse the leeks under running water to remove visible dirt or sand, and try to fan out the tops of the dark green leaves to allow the water to wash away any sandy soil further down the stem. Then cut off the root base and the tough dark green tops and discard - I keep only about 3-5cm of the green. Then place the washed stems in a bowl of water to loosen any further soil trapped, rinse again and drain leeks before slicing finely.