To make the marinade, combine the orange juice, the cumin, the oregano, the achiote paste, the apple cider vinegar and the garlic in a deep bowl. Using a handheld immersion blender, whizz all the ingredients together until the marinade is smooth and creamy.

Using the point of a small paring knife, deeply prick the meat all over so that the marinade can penetrate it. Season the pork well with salt and pepper.

Line a baking dish with a large piece of heavy-duty aluminum foil. Place the meat in the middle of it and bring up the sides to create a well. Carefully pour the marinade over the pork. Scatter the onions over the top. Fold the foil over the meat to make a tight parcel. You might need to use a second piece of aluminum foil to make sure the pork is completely covered. The most important thing is that you seal all of the edges well so that no steam can escape. Let the meat marinate for about 30-40 minutes on the countertop, or up to 12 hours in the refrigerator.

Preheat the oven to 350°. Roast the pork for about 3 to 3 ½ hours. After you remove the pork from the oven, let it rest for about 10-15 minutes before carefully opening the aluminum foil.

While the pork is resting, warm or toast the tortillas on both sides in a large cast-iron skillet set over medium-high heat. Nestle the tortillas in between a napkin on a warmed plate while you finish the rest.

Remove the pork to a cutting board and remove all the strings. Use two forks to gently shred the pork. Pile the shredded pork in a large bowl or dish. Skim as much fat from the surface of the saucy cooking liquid as possible, and spoon as much of it as you want over the pork. Scatter some red onion slices over the top, along with some sprigs of cilantro.

To assemble, pile a good amount of pork onto a warm tortilla. Top it with some of the remaining red onion, a dollop of guacamole and a spritz of lime juice.