Heirloom. A member of the onion family, leeks are often called the gourmet's onion because of their mild flavor. This version, called American Flag, is one of the favorite leeks for home gardens. The mild but flavorful long, leafy leeks are prized for casseroles, stir-fry, soups, and omelets. And, of course, they are a source of flavor and color in the classic leek and potato soup. Plants grow nice, 10-inch-long, white stalks about 1 to 1 1/2 inches in diameter. American Flag is a frost-tolerant, cold-hardy leek, making it good for fall planting in zones 7 and warmer, easily growing where temperatures don't drop below 10 degrees in winter, but they take longer to grow to full size. This also goes by the name Broad London leek.