Preheat the oven to 300˚F.
Pat the pork dry with paper towel, cut it into large (about 2-inch) pieces and sprinkle with salt.
Set a Dutch oven over medium-high heat, add a drizzle of oil and brown the meat well on all sides, sprinkling with chilli powder and cumin.
Transfer to a plate and brown the onion in the pan. Return the pork to the pan and add the tomatoes plus a can of water. (This could all be done in a slow cooker, if you like—cover and cook on low for 6 hours.)
Cover and cook for 3 hours, or until the meat is tender enough to pull apart with tongs or two forks.
Serve with warmed flour or corn tortillas, topped with your choice of toppings.