Place the pork shoulder in a large Dutch oven (or heavy-bottomed pot). Pour in the beef stock. Pour in enough water to just barely cover the top of the pork. Add the onion, garlic, salt, coriander, cumin, and oregano.
Bring the liquid to a simmer over medium heat. Turn the heat all the way low, cover the pot, and gently cook for 3 ½ hours.
Transfer the pork shoulder to a cutting board (optional: strain the liquid, cool it down, and save it to use as a stock). Use two forks to pull the meat apart into smaller pieces.
Heat a 12” cast iron skillet over medium-high heat. Add enough oil to generously coat the bottom. Once hot, add half of the shredded meat in one even layer. Cook for 2 minutes or so until the bottom is crispy. Flip the meat and crisp on the other side for just a minute. Season with a pinch of salt.
Transfer the meat out and repeat with a little more oil and the remaining meat.