tomatillos
A green, tomato-like fruit wrapped in a papery husk, firm and distinctly tart, and the base of many green salsas.
Tomatillos are the fruit of a plant related to physalis. They grow inside a thin, papery husk, which is removed before cooking along with the sticky film on the skin. The flesh is firmer and drier than a tomato's and tastes fresh, sour and slightly grassy. Raw, they give salsas bite and acidity; roasted or simmered they turn softer, milder and almost sweet. Fresh ones can be hard to find outside North America, so look in Latin American shops, at farmers' markets or order by mail. Canned or jarred tomatillos are easier to come by and work well in cooked sauces.
Substitute:
Unripe green tomatoes with a little lime juice come closest in acidity and colour, but the flavour is flatter and less fruity, and the sauce turns out more watery. Physalis are too sweet to serve as a substitute.