We finally tried Tu Y Yo in Somerville (off of Powderhouse Square). It was very good with authentic ingredients you don't usually see around here. The menu leans toward the food south of Mexico City (black beans, masa dough quesadillas, etc.)
The quesadillas were excellent, like corn dough empanadas with various fillings. They had the usual fillings but we tried some that aren't on the website menu: squash blossom and cheese, and the tinga and cheese. Delicious. I also liked the longaniza (somewhat like a fried chorizo) taco so much I ordered longaniza sopes, which I would definitely recommend. The cochanita pibil (braised, shredded pork in a vinegary spicy sauce) was a portion big enough for two and served with black beans and two kinds of rice - yellow and green. Tortillas are good and they even have huitlacoche (corn "mushrooms") quesadillas on the menu - an ingredient I' didn't know you could get outside of Mexico. I've always wanted to try this but alas I was too full. Next time!