Trim the roots from the base of the stalk. Slice leeks lengthwise, from top to bottom, and hold under cold running water with the green end facing down to remove dirt and fine sand trapped in leaf layers. Discard the tough green portion of the stalk unless using it to add flavor to stock or broth. Slice the white portion and sauté with chopped portobello mushrooms and minced garlic, or add to soups and stir-fry dishes.