In a blender or food processor, combine the achiote paste, orange juice, lime juice, white vinegar, minced garlic, oregano, cumin, salt, and black pepper. Blend until smooth.
Place the pork chunks in a large bowl or resealable plastic bag.
Pour the marinade over the pork, ensuring all pieces are well coated. Add the sliced onion to the mixture.
Cover and refrigerate for at least 4 hours, preferably overnight, to allow the flavors to penetrate the meat.
Preheat your oven to 325°F (160°C).
Line a large baking dish with banana leaves or aluminum foil, leaving enough overhang to wrap over the pork.
Remove the marinated pork from the refrigerator and place it in the prepared baking dish, along with the onions and marinade.
Fold the banana leaves or aluminum foil over the pork to seal it tightly.
Cover the baking dish with an additional layer of aluminum foil to ensure a tight seal.
Roast in the preheated oven for about 3.5 to 4 hours, or until the pork is tender and easily shreds with a fork.
Remove the pork from the oven and let it rest for a few minutes.
Shred the pork using two forks and mix it with the juices in the baking dish.
Enjoy your meal!