Leeks are a long, mild member of the onion family, with a white shaft that shades into pale green and then deep green tops. The white part is a growing achievement: farmers hill soil up around each stalk as it grows, blocking the sun so the shaft stays pale and tender, and the higher the hilling, the longer the white portion. That same tight layering is why leeks hide grit: halve them lengthwise and rinse between the layers before cooking. Leeks are gentler and sweeter compared to onions, and cooking makes them gentler still: sliced and softened slowly in butter they turn silky and sweet, the classic start to soups, gratins, and braises. The dark green tops are tough for eating but excellent in stock. They pair especially well with potatoes, butter, thyme, and cream.