Clean leeks thoroughly before using them; they trap a lot of sand and dirt beneath their many layers. First, remove the green tops to within 2 inches of the white part. Peel the outside layer of the bulb, and wash several times in cold water.
Many varieties of sweet potatoes are available. We like the dark orange-skinned sweet potatoes for this dish because they not only make a richer colored dish but also a tastier, sweeter one.