The house trick at many stands is to fill the taco first and then brown the whole thing on the oiled griddle, so the tortilla turns crisp around the stew.
Early-morning stands and small counters across Monterrey set out a dozen or so guisados (barbacoa, machaca with egg, chicharrón in salsa, potatoes, nopales, chorizo) to spoon into flour or corn tortillas, eaten standing on the way to work or class.