Prepare the Chili Sauce: Start by toasting the dried guajillo and pasilla chilies. Heat a dry skillet over medium heat and add the chilies. Toast them for 1-2 minutes, stirring frequently, until they become fragrant but be careful not to let them burn, as this can make the sauce bitter.
Blend the Sauce: Transfer the toasted chilies to a blender. Add the peeled garlic cloves, quartered onion, ground cumin, dried oregano, smoked paprika, and 1 cup of beef broth. Blend the mixture on high until smooth. If the sauce is too thick, add additional broth, a little at a time, until the desired consistency is reached.
Marinate the Beef: Place the beef chunks in a large mixing bowl. Season generously with salt and pepper. Pour the prepared chili sauce over the beef, ensuring all pieces are well-coated. Cover the bowl and let the beef marinate for at least 30 minutes; for a deeper flavor, consider marinating it in the refrigerator overnight.
Cook the Beef: In a large pot or Dutch oven, combine the marinated beef with the remaining 3 cups of beef broth. Bring the mixture to a boil, then reduce the heat to low. Cover the pot and let it simmer gently for 2-3 hours, or until the beef is tender and easily shreds with a fork.
Shred the Beef: After cooking, carefully remove the beef from the pot and place it on a cutting board. Use two forks to shred the beef into bite-sized pieces. Return the shredded beef to the pot, mixing it back into the flavorful cooking liquid, and keep it warm on low heat.
Prepare the Tacos: In a non-stick skillet, heat it over medium heat. Warm each corn tortilla for about 30 seconds on each side until they become soft and pliable.
Assemble the Tacos: Take a warmed tortilla and fill it generously with the shredded beef. Top with freshly chopped cilantro and diced onions for added freshness and crunch.
Serve: Present the tacos immediately, accompanied by lime wedges for squeezing over the top for a zesty kick.
Notes
Serve the tacos stacked on a colorful platter, garnished with extra cilantro and accompanied by a small bowl of the beef broth for dipping.