Place onion slices in the bottom of the slow cooker insert and top with chuck roast.
In a small bowl, mix together all spices (salt through cayenne). Rub mixture evenly all over the beef chuck roast. Pour beer or beef broth on top and cover with lid. Heat on HIGH for 4 hours or LOW for 8 hours.
While beef is cooking, prepare pickled red onions by tossing onion slices with lime juice and salt. Refrigerate for 1 hour or more. The more you stir the onions, the faster they pickle.
When beef is done cooking, shred apart with two forks. Serve on buns with pickled red onions.
Each sandwich should be about ⅓ cup beef and 1 tablespoon pickled red onions.