Leeks are a member of the onion family. Despite sometimes being called "a poor man's asparagus," the thick stalked European leeks commonly found in supermarkets have a mild sweet flavor. Chinese leeks, on the other hand, are smaller and thinner, resembling a thick scallion. Their more pungent flavor makes Chinese leeks are a staple ingredient in northern Chinese cooking. Chinese leeks generally aren't available in North America - regular leeks can be used as a substitute.
Recipes using leeks: