In a large Dutch oven, combine beef chuck roast, lamb shank, and enough water to cover the meat. Bring to a boil, then reduce heat and simmer for 2 hours until tender.

Remove meat, shred it, and set aside. Reserve the broth.

In a skillet, toast the guajillo and ancho chiles until fragrant. Soak in hot water for 15 minutes.

Blend soaked chiles with garlic, cumin, oregano, black pepper, cinnamon, and a cup of reserved broth until smooth.

Combine the blended sauce with the reserved broth in the Dutch oven. Add shredded meat, onion, bay leaves, and salt. Simmer for 1 hour.

Heat corn tortillas on a skillet until warm. Fill each tortilla with shredded meat and top with cilantro and onion.

Serve with a bowl of the broth for dipping and lime wedges on the side.