Trim and clean the leeks: Trim the root end off of the leeks, as close to the end of the leek as possible. For the leeks to hold together while cooking, keep the end of the leek where it all joins together. Trim the stems off of the leek where they turn dark green; the dark green is very tough, and not good to eat. Cut the leek in half lengthwise. Under cold running water, separate layers of the leek and rinse out all the dirt that is trapped inside.
Steam the leeks: Put the halved leeks in the pan in a single layer, cut side down. Add 1/3 cup water and the butter to the pan, then sprinkle the leeks evenly with the salt and pepper. Cover the pan, and put over medium-high heat. Wait for the water to come to a boil, then cook, covered, for 5 minutes.
Saute the leeks: Remove the lid, and check the bottom of the leeks. They should be well browned, and most of the water should be evaporated. (If not, continue to cook cut side down until the leeks are well browned.) Flip the leeks, and cook another 3-5 minutes, until they are tender all the way through. Check for doneness with a paring knife – it should glide through the thickest part of the leek without resistance.
Optionally, top with lemon zest.