This tender, mild member of the lily family looks like an overgrown scallion, but it has a mild, almost sweet flavor that’s all its own. Leeks add a depth of flavor to the stew or soup pot not achieved by onions alone. Served by themselves, leeks can be braised and eaten hot or cold—a favorite first course in French bistros. Sliced and gently sautéed in butter, they make a delicious filling for an omelet or quiche.