Thinly slice the pork: Slice 2 pounds pork butt into 1/4-inch thick strips. If they are really wide, cut them into smaller bite-sized pieces. Place the pork in a glass dish or resealable plastic bag.

Toast the chiles. Heat a large dry frying pan over medium heat. Toast the 6 guajillo chiles and 2 ancho chiles for about 30 seconds per side, until fragrant — don't let them scorch. Fill the pan with water to cover the chiles. Bring to a boil, then remove from the heat and let soak until soft and pliable, about 30 minutes.

Blend the adobada sauce. Remove the chiles with tongs and add to a blender with the 1 cup orange juice, 1/4 cup apple cider vinegar, 4 garlic cloves, 2 chipotle chiles, 1 teaspoon Mexican oregano, 1 teaspoon cumin, 1 tablespoon brown sugar, and 2 teaspoon salt. Blend until completely smooth, 1–2 minutes.

Marinate the pork. Toss the pork strips with the marinade in a large bowl or zip-top bag, coating every piece. Cover with plastic wrap or transfer to an airtight container, and refrigerate at least 4 hours, ideally overnight.

Sear the pork. Heat 1 tablespoon of the oil in a large cast-iron pan over medium-high heat until shimmering. Working in batches so you don't crowd the pan, add the pork in a single layer. Sear without moving for 2–3 minutes until lightly charred. Stir and continue cooking until lightly charred all over but still juicy, stirring occasionally. If the meat is getting too dark before the insides are cooked, lower the heat.

Chop the meat. Remove the meat to a cutting board as it is done cooking, then chop into bite-sized pieces. Repeat with remaining oil and meat, cooking in batches until all the meat is cooked.

Build the tacos. Warm the corn tortillas in a dry pan, 20 seconds per side. Pile on the adobada. Top with white onion, cilantro, avocado, and a squeeze of lime. Serve immediately.