Closely related to tomatoes, these walnut-size green fruits are covered with a papery husk. Although referred to as "green tomatoes" in most of Mexico, tomatillos are more tangy and have more of a citrus flavor than green regular tomatoes. Larger tomatillos have the least flavor and can sometimes be sharp and bitter. Choose small tomatillos that are mature, meaning they have grown to fill their husks. Before using, remove the husks and rinse the tomatillos to remove any sticky residue.