Prepare the Chiles: In a dry skillet over medium heat, toast the guajillo and ancho chiles for about 1-2 minutes until slightly fragrant. Be careful not to burn them. Place the toasted chiles in a bowl and cover with hot water. Let them soak for 15 minutes, then drain.
Blend the Marinade: In a blender or food processor, combine the soaked chiles, garlic, cumin, oregano, smoked paprika, and apple cider vinegar. Blend until smooth, adding a little beef broth if needed to get a smooth paste.
Sear the Beef: Season the chuck roast with salt and pepper. Heat a skillet over medium-high heat and sear the beef on all sides until browned (about 3-4 minutes per side). This step is optional but adds extra flavor.
Crockpot Layering: Place the chopped onion at the bottom of the crockpot. Top with the seared beef chuck roast and spoon the blended sauce over it. Pour in the remaining beef broth.
Cook: Cover the crockpot and cook on low for 8 hours or high for 4 hours, until the beef is tender and shreds easily.
Shred the Beef: Once cooked, remove the beef from the crockpot and shred it using two forks. Return the shredded beef to the crockpot and mix it with the sauce.
Assemble the Tacos: Heat corn tortillas on a skillet or directly over the flame for a few seconds until warm. Fill each tortilla with the shredded birria and top with chopped cilantro.
Serve: Serve the tacos with lime wedges on the side and a small bowl of the reserved cooking liquid for dipping.
Notes
Enjoy the tacos with a side of pickled onions or a fresh salsa for added flavor.