To prepare leeks: slice off the ends with the roots and discard those. Slice off the green portion and save those for another recipe; for this recipe, we're just using the white portion. Slice the white portion of the leek in half lengthwise, then slice those thinly so you end up with half-moon shaped thin slices. Place the leek slices in a collander and rinse with cold water. Leeks can hold a lot of dirt and this helps clean them out before cooking.