Toast and soak the dried chiles: Heat a dry skillet over medium heat and toast the dried guajillo and ancho peppers for 20–40 seconds per side until fragrant. Place the toasted peppers in a bowl and cover with very hot water to soak for about 15 minutes until soft.

Make the sauce: Drain the softened chiles and blend with beef broth, chopped onion, minced garlic, cumin, oregano, and salt and pepper until smooth. Adjust seasoning as needed.

Brown the meat (optional): Season the beef with salt and pepper and sear each side in a hot skillet for 2-3 minutes until browned.

Combine meat and sauce in the Crock Pot: Place the meat in the Crock Pot and pour the blended chili sauce over the top, ensuring it's mostly submerged. Cover and set to cook.

Cook on LOW for 8 hours or HIGH for 4 hours, until the meat is fork-tender.

Remove the meat, shred it using two forks, and return it to the Crock Pot. Let it sit on warm for 15-20 minutes.

Warm corn tortillas and fill each with shredded birria, garnishing with chopped onions and cilantro.