In a dry skillet over medium heat, toast the guajillo and ancho chilies for a couple of minutes until fragrant. Be careful not to burn them. Remove from heat and soak the chilies in hot water for about 15 minutes until soft.
In a blender, combine the soaked chilies, onion, garlic, cumin, oregano, cinnamon stick, apple cider vinegar, and a pinch of salt and pepper. Blend until smooth. You may need to add a little broth to help with blending.
Place the chunks of beef in the crockpot. Pour the blended chili sauce over the beef and add the beef broth. Stir to combine, ensuring the beef is well-coated in the sauce.
Cover the crockpot and cook on low for 8 hours or on high for 4-5 hours, until the beef is tender and easily shreds with a fork.
Once cooked, remove the beef from the crockpot and shred it with two forks. Return the shredded beef to the crockpot to soak in the juices.
In a separate skillet, lightly toast the corn tortillas over medium heat for 30 seconds on each side. Add a generous amount of the birria beef to each tortilla.
Top with chopped cilantro, serve with lime wedges, and sliced radishes on the side.
Notes
For extra flavor, dip the tacos in the birria broth before eating.