For the filling
1 lb. trimmed beef shoulder
3 cloves garlic
1 bay leaf
1 large white onion, half coarsely chopped
1⁄4 cup canola or rice bran oil
1⁄4 cup dried árbol chiles, stemmed and seeded
4 large dried guajillo chiles, stemmed and seeded
2 medium tomatillos, husked and rinsed
For the empanadas
2 cups Homemade Masa, or masa prepared from store-bought masa harina
2 tbsp. lard
1⁄2 tsp. baking powder
Canola oil or rice bran oil, for frying Your choice of garnishes such as sliced green cabbage, cilantro, hot sauce, crumbled queso fresco, and sour cream
Make the filling: Place the beef in a medium pot and season all over with salt. Add 2 garlic cloves, the bay leaf, the unchopped half of the onion, and enough water just to cover the meat. Bring to a boil, then reduce to a simmer and let cook until tender, 1 1⁄2-2 hours. Let cool slightly. Finely shred the meat and season generously with salt (discard the broth).
Meanwhile, in a large skillet heat the oil over medium-high heat until hot. Add all of the chiles and cook, turning frequently with tongs, until bright red and lightly toasted in places, 15-20 seconds (do not overcook or chiles will be bitter); remove and transfer to a heatproof bowl. Cover with boiling water and let sit until softened, 20 minutes.
Transfer the chiles to a blender, adding only a little of the soaking water as needed to help blend. Add the remaining garlic clove, the tomatillos, the chopped onion, and a generous pinch of salt and purée until smooth. Combine the salsa with the shredded beef, starting with just enough to moisten the meat and adding more to taste; do not overmoisten. (Meat and salsa can be prepared up to 2 days ahead.)
Make the empanadas: In the bowl of a stand mixer fitted with the paddle attachment, mix the masa with the lard and baking powder until well combined. (Alternatively, you can stir the ingredients by hand in a large bowl, but the mixer will produce lighter, airier empanada shells.)
To form the empanadas, line a tortilla press with two small rounds of parchment paper or plastic cut from a plastic bag to fit. Scoop out 1⁄4 cup of masa dough and form it tightly into a ball, then flatten the ball slightly into a tight disk. Place the disk in the center of the tortilla press, sandwiched between the two rounds of plastic. Close the press to form a large round slightly thicker than a tortilla (about 1⁄8 inch thick when raw; the dough will expand in the fryer). Carefully peel away the top liner. Holding the pressed masa round in one hand atop the plastic round, place some of the beef filling (a scant 1⁄4 cup) at its center, leaving a generous border. Using the remaining piece of plastic to support the delicate masa, carefully fold the empanada shell in half to cover the filling and form a half-moon shape. Press the joined edges of the pastry together to seal tightly. Transfer to a parchment paper-lined baking sheet or platter and repeat with the remaining masa and filling.
Set a plate lined with paper towels next to the stove. In a 12-inch skillet, add enough oil to come 1 inch up the sides of the pan. Heat the oil until a deep-fry thermometer reads 350° or a small piece of masa dropped into the center bubbles vigorously. Working in batches of 2-3 as needed, carefully place the empanadas in the pan; cook until lightly browned, about 4 minutes, then turn and repeat. Transfer to the prepared plate and season with salt. Serve garnished with your choice of cabbage, cilantro, hot sauce, queso fresco, and sour cream.