Leeks are like the Lord Thistlewick Flanders of the onion family. The refined and aloof European cousin who needs to be nudged before his true onion character emerges. But once you start cooking with them, they offer a variety of characteristics that you don't find in regular onions.
First off, there's texture. When you cook down a regular onion, it becomes very soft, or it completely disintegrates. Leeks, on the other hand, retain a bit of tender bite, acquiring a pleasing texture more similar to that of, say, cooked cabbage. This property makes them fantastic for whenever you want to add nice, mild onion flavor without the stringiness or pulpiness of regular onions. Cooked very slowly in emulsified butter (often described on menus as "leek fondue"), they are incredible.
Flavor-wise, they're far more mild than a regular onion, without the sweetness of a shallot. Slightly grassy and vegetal, they're known as the "soup onion" for a reason—they're great in soups and stews. Their flavor tends to sit back and support other flavors without overwhelming them.
Leeks are also quite versatile. They can be stewed, braised whole, roasted, sautéed, and, as any chef from the early '90s can tell you, deep-fried into crisp frizzles to add texture to soups and salads (though your soup will end up looking like Don King).
Shopping and Storage
Look for leeks with tight, bright-green tops that aren't wilting at all. The white bases should be bright white—yellowing is a sign of age. At the supermarket, most leeks these days are pristine, with no dirt at all. But if you do find muddy leeks, that's okay, and should be expected—leeks grow pretty deep in sandy soil. Just be sure to carefully wash away all grit and sand before cooking with them (this is addressed in the above video).
Since they don't have the robust dried exterior of a standard onion, the shelf life of leeks is far shorter. Leeks can be stored in a plastic vegetable bag in the crisper drawer for about two weeks or so, though one week is more normal. Generally, I buy them as I need them instead of treating them as a staple to always have on hand.
Only the tender white and light-green bottoms are actually eaten, but the fibrous green upper parts still have plenty of flavor and can be added to the pot next time you make stock.
How to Cut Leeks: Step by Step
Step 1: Trim Root End
Slice off the root end of the leek, being careful not to cut off any more of the tender white portion than necessary. If you don’t cut off the root fully, though, it will hold some of the layers together, making it difficult to separate them later. You’ll know if you got the root completely off if you can see every concentric ring of the leek’s layers, all the way from edge to center.
Step 2: Trim Tough Green Tops
Trim off the tough dark-green tops. These are usually too fibrous for cooking and eating, but they have great flavor and can be saved for making stocks, soups, and broths.
Step 3: Remove Damaged Outer Layers
Unless you’re buying your leeks fresh from a farmer who dug them up that very morning, chances are they’ve been banged around a bunch during transit and on market shelves. They’ve also probably spent enough time out of the earth that the outer layer or two will have begun to dry out. Trim off any outer layer that seems withered, damaged, or otherwise beyond salvation. (These, too, can be good for stocks and such.)
Step 4: Cut Into Manageable Lengths
Cut each leek into more manageable four- or five-inch lengths.
Step 5: Cut in Half Lengthwise
Then slice each piece in half lengthwise to form perfect half cylinders.
Step 6: Wash Very Well
Wash each half cylinder of leek under running water, making sure to peel back the layers. Leeks have a tendency to pick up dirt and sand in their layers as they grow, so you need to be vigilant about washing them well—even a few grains of sand are enough to ruin a dish.
In addition, make sure to wipe down your cutting board before returning your washed leeks to it, lest any sand that fell off before get back on them.
How to Cut Half Moons
Cross-cut each half cylinder to make half moons. Vary the thickness as desired.
How to Cut Strips
Peel off a few layers from a portion of the leek, stack them, then cut lengthwise into strips. Continue with the remaining layers of leeks.
How to Cut Dice
First cut strips as described above, then cross-cut into dice.
How to Cut Julienne
Stack a few layers of leeks on top of each other, fold them in half, then very thinly slice them into fine strips.