By Harold McGee
Tomatillos are the fruits of Physalis ixocarpa, a tomato relative that in fact was cultivated before the tomato in Mexico and Guatemala, to whose cool highlands it is better adapted. The tomatillo fruit is smaller than a standard tomato but similar in structure, and is borne on the plant enclosed in a papery husk. Its skin is thick and tough, sticky with a water-soluble secretion (the plant’s species name, ixocarpa, means “sticky fruit”), that helps it keep well for several weeks. The tomatillo remains green when ripe and has a tart but otherwise mild, green flavor, a relatively firm and dry texture. It’s usually cooked and/or pureed into sauces, with other ingredients adding depth or strength of flavor. A related species, P. philadelphica, provides an esteemed purple version called miltomate.