Sometimes called Mexican green tomatoes, the tomatillo looks like a small green tomato with a papery covering or husk. It is in the same family as a tomato, but tomatillas have a distinctive tart flavor. For a tomatillo substitute, buy underripe tomatoes and add a squeeze of lime juice.
You can buy tomatillos in Latin-American markets year round as well as in many specialty stores and some supermarkets. The papery covering is removed before cooking.