Tomatillos, also known as husk tomatoes because of the papery brown husk that covers them, are small, tart and green. Their colour is not, however, from lack of maturity; they are ripe when green. Tomatillos give an unusual tanginess to whatever they are cooked with, and are the basis of my favourite Mexican salsa.
They are commonly sold in tins throughout the Western US; if you should find some in the UK, snap them up. You can order them through Mexicolor