1Pat the chuck roast dry with paper towels, then cut into 3-inch chunks—don't worry about perfect pieces.
2Toast the guajillo and ancho chiles in a dry skillet over medium heat for 30 seconds on each side until fragrant, then remove stems and seeds.
3Blend the toasted chiles, achiote paste, minced garlic, apple cider vinegar, cumin seed, oregano, black peppercorn, and salt with 1/2 cup of the beef broth in a blender until smooth.
4Add the beef chunks, halved onion, bay leaves, and juice from both limes and the orange to your slow cooker.
5Pour the chile-achiote mixture over the beef and add the remaining 1/2 cup broth. Stir well to coat everything.
6Cover and cook on low for 8 hours, or until the beef shreds easily with a fork.
7Using tongs, transfer the beef to a cutting board and shred with two forks into bite-sized pieces.
8Discard the bay leaves and skim any excess fat from the cooking liquid, then pour about 1 cup of the liquid back over the shredded beef to keep it moist and flavorful.
9Warm the tortillas in a dry skillet or over a gas flame until pliable. Spoon a little of the cooking liquid back over the beef, then build each one with shredded barbacoa, sliced white onion and cilantro.