Tacos al Pastor

Place the guajillo and ancho chiles in a small saucepan, cover with 1.5 cups water, and simmer over medium heat for 8 minutes until softened. Drain, keeping 1/2 cup of the soaking liquid.
In a blender, combine the softened chiles, 1/2 cup soaking liquid, pineapple juice, vinegar, garlic, cumin, oregano, and salt. Blend on high for 1 full minute until completely smooth with no visible chile skins.
Put the sliced pork into a large zip-top bag or bowl, pour the marinade over it, and massage to coat every piece. Refrigerate for at least 3 hours, or up to overnight for deeper flavor.
Remove the pork from the fridge 20 minutes before cooking so it is not ice-cold going into the pan.
Heat 1 tablespoon of oil in a large cast-iron or heavy skillet over medium-high heat until it shimmers, about 2 minutes.
Lay pork slices in the skillet in a single layer, working in batches so you do not crowd the pan. Cook 3 to 4 minutes per side until deeply browned with charred edges and the internal temperature reads 145 F on an instant-read thermometer.
Transfer the cooked pork to a cutting board and let it rest for 5 minutes, then chop it into small, bite-sized pieces.
Wipe the skillet clean, add the remaining 1 tablespoon oil, and saute the diced pineapple over medium-high heat for 3 minutes until lightly caramelized at the edges; set aside.
Warm the corn tortillas one at a time in a dry skillet over medium heat, about 20 seconds per side, until pliable and lightly speckled, then stack them in a clean kitchen towel to stay warm.
Double up two tortillas per taco for sturdiness, spoon on a generous portion of chopped pork, then top with caramelized pineapple, chopped onion, and cilantro.
Serve immediately with lime wedges for squeezing over the top; do not let the tacos sit assembled for more than a few minutes or the tortillas will get soggy.

Slicing the pork thin (1/4 inch) before marinating lets the chile paste penetrate fully and helps it char quickly in the skillet like true al pastor.

If you cannot find guajillo or ancho chiles fresh, toast dried ones in a dry pan for 30 seconds per side first to wake up their flavor before soaking.