Leeks is one of the sweeter members of the onion family, as it looks like an overgrown scallions. Leeks are available year-round, so leeks tend to be best from the fall until the spring, when their cores begin to turn tough. Look for those with bright, crisp leaves, and buy the smallest ones in the market, which tend to be sweeter.
Store leeks in a perforated plastic bag and refrigerate for up to 1 week. When washing leeks, be careful as leeks often hide a great deal of grit in their many tight leaf layers. The easiest way to wash them is to slice or chop them first, then place the pieces in a large bowl. Fill the bowl with cool water. Swish the leeks about with your hands to loosen any dirt, which will settle at the bottom of the bowl. Lift out the cleaned leek pieces with a slotted spoon or small strainer. Avoid pouring the leeks and water from the bowl through a strainer of colander, which could cause the dirt to fall back onto the cleaned leeks.
When slicing leeks, trim off the root and the dark green leaves at the top of the leek, leaving about 1" of the pale green part attached to the white. Cut the leek in half lengthwise, and then cut crosswise into thin slices.