If the outer skin in dry (or slimy) just peel it off.
Leeks are a staple in my household. They are a sturdy, versatile, flavorsome vegetable with many uses, and you will always find a few lurking in my produce drawer. A real kitchen workhorse, they can be blanched, steamed, braised, or grilled and then used in soups, stews, omelet fillings, or even confit. But sometimes it can be confusing to encounter a leek. How much of the greens should one cut off? What's with all that grit?
Leeks are a member of the Allium family, just like garlic, onions, and lilies. They are used in recipes when a milder onion flavor is called for, although they will stand in for onions just fine. Unlike onions, however, leeks don't caramelize very well: instead of turning sweet, they become bitter when browned.
When choosing leeks, look for ones that have a lot of white and pale green, which is the tender, edible part. This is achieved by growing the leeks in trenches and then burying the stalks in dirt as they grow so they blanch and stay tender. This explains why leeks are often quite dirty, although frankly they seem to be much cleaner these days. The tough darker green sections are not edible, but they are an excellent ingredient for making stock and should not be discarded.
There are two common ways to use leeks: cut in half lengthwise and baked or blanched (such as leeks vinaigrette) or chopped up as an addition to a soup or stew. Both prep methods are covered below.
How Wash and Prep Leeks
What You Need
Large sharp knife
1. Remove the tough tops. If the first layer of skin is dry or slimy, peel it off and discard. Remove the tough darker green leaves by slicing the leek crosswise at the place where the pale green color ends and the darker green begins. It's a little tricky to figure out exactly where to do this but there is a definite difference between the thin, tender, pale green and the tough, thick, darker green sections of the leek. You can always trim off some of the tough bits later if they don't make the first cut. Conversely, you can also rescue some of the tender green parts out of dark leaves.
→ Keep the tough tops! Wash them well and use them for stock.
2. For halved leeks. If you will be using whole leek halves in your recipe, simply cut them in half lengthwise. Do not cut off the root at this point: the root will hold the leeks together when you wash them. Wash them under cool running water, using your fingers to gently fan the layers and dislodge any grit. Drain in a colander.
3. For chopped leeks. If your recipe calls for chopped leeks, it's easier to chop the leeks first and then wash away the dirt. Cut the leek in half lengthwise. If your leek is especially fat, or if you would like smaller pieces, cut these halves into quarters. Start by slicing off the root end (which can go with the tough upper leaves and used for stock). Then slice each half crosswise into half moons, working your way up the leek until you reach the green leaves at the end. If you encounter some remainders of the tough green, just discard that part.
Place the sliced leeks in a colander and wash under cool running water until grit-free. Drain before using.
(Images: Dana Velden)