Preheat the oven to 300˚F.
Set a shallow pot or Dutch oven over medium-high heat, add a drizzle of oil and brown the meat on all sides, seasoning it with salt and pepper. Transfer to a plate and add the onions to the pot; cook for a few minutes, loosening any browned bits in the bottom of the pan. Stir in the garlic.
Return the pork to the pan, add the oranges and half the cilantro and pour in enough stock, beer or water to come about halfway up the side.
Cover and cook for 3 hours, or until the meat is very tender. 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.
Serve on tortillas topped with feta, avocado, cilantro and lime wedges for squeezing overtop.