I stumbled across a truly authentic Mexican place in the Fitchburg area last week:
El Volcan Mexican Restaurant & Bar
5 Summer St.
Lunenburg, MA 01462
The food is Colima style (homesick Angelenos take note!), so that means salsa made with red chilies rather than tomatoes, and very spicy sauces. I had chili colorado (beef chunks in red sauce), which was made from steak and excellent. I held a fork in one hand and a napkin to dab my brow with the other. Homemade chips as well. The place was deserted last Thursday night, so please, anyone, go there! : ) Aside from cold Coronas, they also have jamaica and at least occasionally horchata, though not when I was there.
Incidentally, there are 2 nearly identical Asian food marts in the same strip mall, one Hmong and the other Vietnamese-Laotian. Apparently, the latter signed the lease without realizing the former was already there (at least that's what he told me).