Leeks are notorious for harboring grit within their layers; they should be cleaned thoroughly before cooking.
Trim the top of the leek, leaving some green, or removing it altogether depending on the recipe. Discard the outer leaves and trim the root. Split the leek in quarters or halves almost through the root, depending on size.
Rinse the leek down to the root in cold water, shaking it to loosen any dirt. Reassemble the layers for cooking. Note If the leek is to be sliced or chopped, it may be fully quartered or halved before cleaning.