In a bowl, combine the garlic, lime zest and 1/4 cup lime juice (reserving the other ¼ cup for later), orange zest and juice, salt, pepper, oregano, and cumin and whisk together until well combined. Using a paring knife, cut 1 inch deep slashes into the six pieces of pork, placing them into a large mixing bowl or a large freezer bag as you go.
Evenly pour the marinade over the pork and give it a good mix to thoroughly coat all the pieces of pork and get into all the crevices. If using a bowl, cover with plastic wrap and if using a bag, seal it and then transfer to the refrigerator to marinate for six hours.
After the pork has marinated for six hours, set your pressure cooker to the sauté setting on high and add the olive oil. Add three of the six pieces of pork to the cooker, shaking off any excess marinade back into the bowl or bag, and brown for three minutes on each side. Place the browned pieces on a plate and repeat the process with the remaining three pieces of pork.
Transfer the browned pieces on the plate back to the cooker and add any remaining marinade juices from the bowl or bag into the pot. Put half of the onion slices, reserving the other half for later, on top of the pork and Lock the lid. Cook on high pressure for 1 hour and 15 minutes, letting the pressure release naturally when it’s done.
Transfer the pork pieces to a serving platter and use two forks to shred the meat. Taste a piece and sprinkle some additional salt and pepper if necessary. Cover with a piece of foil and rest. Using the simmer function on your cooker, add the reserved 1/4 cup of lime juice and reserved onion slices and simmer them with the juices for 10 to 15 minutes.
Taste the juices from the pot and see if you need to adjust with more salt and pepper. Toss the shredded pork with about a cup or so of the juices and the simmered onion slices. Serve with lime wedges, cilantro, rice, sweet plantains, and black beans.