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. Add the stock—enough to come about halfway up the side of the meat.
Cover and cook for 3 hours, or until the meat is tender enough to pull apart with tongs or two forks. Alternatively, put the pork shoulder, spices and stock into a slow cooker and cook on low for 6 hours.
Meanwhile, stir together the refried beans and green onions.
Heat about an inch of canola oil in a heavy pot until hot but not smoking; cook the corn tortillas (as many as you want to serve) for 1-2 minutes, turning with tongs, until golden and crisp.
Set aside on a paper towel-lined plate to drain, then spread with refried beans, top with pulled pork and your choice of toppings.