Preheat the oven to 300F.
Set a Dutch oven over medium-high heat, add a drizzle of oil and brown the meat on all sides, seasoning it with salt and pepper, letting it sit and develop a nice crust on each side. Transfer to a plate and add the onions to the pot; cook for a few minutes, loosening up any browned bits in the bottom of the pan. Stir in the garlic.
Set the pork on top of the onions and pour in enough beer or stock to come about halfway up the side. Tuck in a few halved mandarin oranges, squeezing some of their juice over the meat first, if you like. Toss in a handful of cilantro (or not).
Cover and cook for 3-5 hours, or until the meat is very tender. If you like, remove it from the oven and prod the meat a bit or flip it over halfway through. Spoon off any excess fat (or if you like, cool and refrigerate, then lift off the excess fat before reheating). Shred with forks, tossing with the juices and broken-down onions and oranges. Add more salt, if it needs it.
Serve as-is, or stuff soft tortillas and add crumbled feta, avocado and cilantro. Serves lots.