Tomatillos are more closely related to gooseberries than to regular tomatoes. They are not underripe tomatoes, but a separate fruit entirely. They have long been popular in Mexican cooking, where they are usually lightly stewed and flavored with various combinations of garlic, peppers, onions, and cilantro.
Tomatillos can be prepared in much the same way as tomatoes, except that their paper-like sheath should be removed and they should be lightly cooked, as they are usually too tough to eat raw. They can be gently stewed and puréed into a colorful tangy sauce that can be flavored with traditional Mexican flavors or adapted in other ways.