Place the beef chuck roast in the slow cooker. Pour in the beef broth, then add the chili powder, cumin, smoked paprika, garlic powder, salt, and black pepper. Stir the spices around the meat to coat it evenly.
Cover the slow cooker and cook on low for 8 hours until the beef shreds easily with a fork.
About 30 minutes before the beef finishes, thinly slice the red onion. In a mixing bowl, stir together the apple cider vinegar, sugar, and a pinch of salt. Add the onion slices and press them down so they sit in the liquid. Place the bowl in the fridge to pickle.
Once the beef is done, shred it directly in the slow cooker using two forks and mix it with the cooking juices.
Warm the corn tortillas. Fill each tortilla with shredded beef, then top with pickled onions, chopped cilantro, and a squeeze of lime.