Heat 2 tablespoons of oil in a large skillet set over medium-high heat. Add the onion and serrano and cook for 3 to 4 minutes, until wilted. Stir in the garlic, stir and cook for a minute, until fragrant but not burnt. Add the tomatoes, tomatillos, and salt and cook stirring often, until they have completely softened and released their juices and then begin to dry up turning into a moist but mushy paste, about 7 to 8 minutes. Turn off the heat. Add the queso fresco and combine well. Set aside.
To assemble and dress the tacos:
Set your comal, cast iron, or nonstick skillet over medium heat. If desired, add a tablespoon of oil and spread throughout (you can also make the tacos without oil). Place a couple tortillas and add a couple tablespoons of the queso filling in the center of each. Fold the tortillas to close, and let the tacos cook and begin to crisp for a couple minutes on each side. Repeat with all the tortillas.
Serve, completely sauced with the Salsa Verde con Piquín. You may drizzle on Mexican crema and sprinkle some crushed dried Piquín on top.