A tomatillo is a fruit that belongs to the same poetically titled "nightshade" family as the tomato. Also called a "Mexican green tomato," it does look like a small green tomato with a papery husk in place of skin. But it has a tart flavor all its own.
A key ingredient in Mexican and Latin American dishes such as guacamole and salsa verde, the tomatillo's flavor is deepened by cooking.
While tomatillos can also be purchased canned, fresh, ripe tomatillos will keep in the refrigerator for about two weeks (or can be frozen). Tomatillos turn yellow as they continue to ripen, but should be used while still green in order to take advantage of their tart flavor.