Prepare the Chiles: In a dry skillet over medium heat, toast the guajillo and ancho chiles for about 2-3 minutes until fragrant. Be sure not to burn them. Once toasted, transfer to a pot and cover with hot water for about 15-20 minutes to rehydrate.
Make the Sauce: Drain the chiles and add them to a blender along with onion, garlic, cumin, oregano, black pepper, and a splash of beef broth (about 1 cup). Blend until smooth, adding more broth if necessary.
Sear the Meat: In a large pot or Dutch oven, heat a little oil over medium-high heat. Season the beef chunks with salt and brown them on all sides (about 5-7 minutes).
Cook the Meat: Pour the blended sauce over the browned beef, add the remaining beef broth, bay leaves, and stir well to combine. Bring to a simmer, then cover and reduce heat to low. Cook for about 2-3 hours until the beef is tender and shreds easily.
Shred the Meat: Once cooked, remove the beef from the pot and shred it using two forks. Discard bay leaves and reserve the cooking liquid.
Heat the Tortillas: In a separate skillet, warm the corn tortillas over medium heat for about 30 seconds on each side until pliable.
Assemble Tacos: Place a generous amount of shredded beef in the center of each tortilla. Top with chopped cilantro and onions. Optionally, drizzle with some of the broth for extra flavor.
Serve: Serve the tacos with lime wedges on the side for squeezing over the top.