Leeks are root vegetables that look quite similar to onions, to which they are related. Their flavor is onion-like but much milder. People who avoid this vegetable because they don't like onions should try them — their flavor is mellow and not overpowering, and many onion-haters enjoy them.
Unlike onions, leeks don't form much of a bulb on the end of the root. Instead, they remain cylindrical, with perhaps a slight bulge at the end. The part of that is under ground remains tender and white, while the part exposed to the sunlight becomes tough and fibrous and not very good for eating. To maximize the edible part of the plant, farmers mound the dirt up around the sprouting plant; this keeps more of it underground and white, but also means that dirt often gets between the layers, so leeks need careful cleaning before cooking.
Leeks are most commonly used in soup, most notably in vichyssoise, a cold soup that also contains potatoes. Cooks who have a favorite potato soup recipe might want to try adding some sliced leeks next time they prepare it, since the flavors go well together. A combination of leeks, potatoes, and carrots in a chicken broth makes a dish that many people enjoy. The vegetable is also edible raw, and it can impart a great crunchy flavor to salads or when eaten with a dip. It should be cut in half lengthwise and rinsed thoroughly to remove and dirt or grit, then it can be added to a platter of crudites.
Nutritionally, leeks are a great source of fiber and may actually help lower cholesterol. They're also packed with important vitamins and minerals, including potassium.
According to Welsh tradition, back in the days before military uniforms, the Welsh fighters were instructed by their king to distinguish themselves from the enemy by fixing a leek to their helmets. Whether because of this legend, or for older reasons, the plant is one of Wales' national symbols, and it is worn on the lapel in honor of St. David, Patron Saint of Wales, on his Day.