Preheat the oven to 300˚F.
Drizzle some oil into a heavy, ovenproof pot set over medium-high heat.
Brown the pork in batches, sprinkling with chili powder, salt and pepper and transferring it to a bowl or plate as you go.
Remove from the heat, add orange juice and water, cover and braise for 3 hours, or until the meat is very tender.
Remove from the oven and shred with forks.
Meanwhile, roughly mash the beans, seasoning with the rest of the chili powder, salt and pepper.
Spread tortillas with the beans, top with pork and serve with chopped onion, tomato and avocado, salsa and sour cream, with lime wedges for squeezing overtop.