Heat the vegetable oil in a heavy-bottomed Dutch oven over high heat. The oil should shimmer, and the pork should give a clear sizzle when it touches the pan.

Season the pork shoulder with the kosher salt, then brown it on every side in the hot oil. Turn the pork until a deep golden-brown to mahogany crust forms across the surface.

Stir the beef broth, diced onion, minced garlic, chipotle peppers in adobo sauce, lime juice, chili powder, brown sugar, cumin, paprika, dried oregano, and cayenne together in the slow cooker. The liquid should smell smoky, savory, and slightly bright from the lime.

Set the browned pork into the seasoned liquid and cook on low for 8 to 10 hours. The pork is ready when a fork twists through it with almost no resistance and the meat separates into tender strands.

Lift out the pork, remove any large pieces of fat, and shred the meat with two forks before returning it to the slow cooker. Toss the shreds through the cooking juices until they look moist and evenly coated.

Fill corn tortilla shells with the pork and top with cotija cheese, pickled red onions, and cilantro. Build each taco while the pork is hot so the cheese softens slightly and the herbs stay fresh.