Leeks are root vegetables that belong to the genus Allium, which also includes onions, garlic and scallions. They have a mild, onion-like flavor when prepared and have been likened to giant scallions in appearance. The white base of the vegetable is the edible part, and the leaves and roots must be removed prior to cooking. When choosing leeks to prepare, select those with dark green, stiff leaves and smooth bottoms.
Place the leeks on a cutting board or another hard, flat surface. Use a sharp knife to cut off the green leaves at the top, leaving behind about 1 inch of green above the white part of the stalk. Cut off the fibrous roots from the bottom of the leeks.
Discard the leek leaves and roots, and then remove the outermost layers of the remaining white stalks. Use the knife to gently peel or whittle away the first few layers, leaving behind only the desirable inner parts of the leeks.
Cut the leeks in half lengthwise, and then dice them into 1 to 2 inch pieces. Proper cleaning is necessary at this stage, as dirt can easily become trapped between the leaves. Transfer the leek pieces to a large bowl filled with cold water.
Swish the pieces around with your hands to remove any dirt or debris. Allow the leeks to remain in the water for 2 to 3 minutes, and then carefully remove them with a slotted spoon to prevent stirring up the dirt that's fallen to the bottom of the bowl.
Cook leeks as usual once they've been thoroughly cleaned. Try boiling them in water and serving with melted butter or olive oil, or use them in recipes as desired. Leeks are also commonly served cold and tossed with vinaigrette dressing.