Cut open the dried Guajillo peppers and Ancho chilies. Remove stems and seeds, then cut into smaller pieces. Place in a bowl, cover with boiling water, and let rest for 10 minutes. Drain and set aside.

Preheat a large skillet over medium-high heat. Cut the chuck roast into four equal pieces. Sear each piece for 3 to 4 minutes on each side until browned. Transfer the browned pieces to the slow cooker.

In a food processor, combine the rehydrated peppers, garlic cloves, one onion, tomato sauce, oregano, cumin, kosher salt, and apple cider vinegar. Blend until smooth.

Pour the sauce over the browned meat in the slow cooker. Add bay leaves and beef broth, then stir to combine. Cover and cook on low for 7 to 8 hours.

Once cooking is complete, skim off the oil floating on the surface of the broth using a ladle.

Carefully remove the beef and shred it into bite-sized pieces. Ensure each piece is coated in the flavorful sauce.

Heat a skillet or flattop grill over medium heat. Dip one side of a tortilla into the reserved oil, then place it oiled side down in the skillet. Top with cheese and shredded beef, cooking until crispy.

Open each taco and sprinkle with diced onion and cilantro before closing. Serve with a side of reserved broth for dipping.