a stovetop pressure cooker; a deep-fry thermometer; toothpicks
For the arbol salsa: Preheat the broiler to high.
Place the tomatillos, tomatoes and garlic on a sheet pan and drizzle with the oil. Toss to combine, then place under the broiler and broil, turning occasionally, until the tomatillos and tomatoes are slightly blackened, about 10 minutes.
Transfer to a medium saucepan with 1/2 cup water and turn the heat on to medium. Bring to a boil, then add the chiles de arbol, vinegar and salt. Turn to medium-low, then cover and cook for 20 minutes. Place in a blender and blend to smooth, then taste and adjust salt as needed. Reserve in a bowl.
For the beef cheeks: Place the beef cheeks, guajillo chiles, oregano, garlic, bay leaf, white onion, 1 tablespoon salt and 2 cups water in a stovetop pressure cooker. Seal the lid and bring up to high pressure. Reduce to medium pressure and cook for 45 minutes. Carefully remove the pressure cooker from the heat, then allow the pressure to subside and carefully remove the beef cheeks to a cutting board. Discard the aromatic ingredients. Cut the beef cheeks into small pieces. Taste and adjust seasoning with salt.
To build the tacos: Heat a griddle or 14-inch cast-iron pan over medium-high heat. Working 3 at a time, place 3 tortillas on the griddle and heat until pliable, 1 minute on each side. Transfer to a cutting board and put 2 tablespoons chopped beef cheeks in the center of each tortilla. Fold over and secure with a toothpick by threading through to close the tortilla. Repeat until you have formed 18 little tacos. (You may have extra tortillas; these are backup for the ones that might tear.)
Place the vegetable oil in a medium pot and bring up to 350 degrees F. Line a sheet tray with paper towels.
Add the formed tacos, 3 to 5 at a time, and fry, flipping halfway with tongs, until golden brown and crispy, 2 to 3 minutes. Remove to the paper towels to drain and continue cooking the remaining tacos. Place the tacos on a platter with the bowl of salsa.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.