Commonly called tomate or green tomato, they have nothing in common with tomatoes. Tomatillo is a fruit of the same family as Physalis, characterized by the husk that is surrounding it.
It is quite difficult to find fresh outside of Mexico as they are a Mexican indigenous ingredient. I have only found it canned in Paris some years ago. Fortunately I have found some fresh one some weeks ago in a gourmet store in Oslo: Maschmanns (which I advice you to visit if you have the chance, you will find there the best Norwegian delicacies among others from all around the world).


