The covered market (Mercado Municipal) near Tehuacan's zócalo is the most honest place to eat in the city. The prepared food section runs along the interior, with stalls selling cecina platters — the local dried and salted meat specialty — alongside tasajo (a Oaxacan-style dried beef), chorizo, and memelas. A full mixed platter with tortillas, salsa, and beans costs very little and is more interesting than anything you'll find in the restaurants around the main square.
The cecina style of Tehuacan is slightly different from the Oaxacan version: the meat is cut thinner and the drying is more pronounced. It is eaten throughout the region and is the local taste you'll miss if you skip the market. The market opens early and is at its best for food in the late morning.