Preheat a large skillet over medium-high heat. Add the vegetable oil and sear the chunks of beef chuck roast on all sides until browned. Transfer the beef to a slow cooker.

In the same skillet, toast the guajillo, ancho, and chipotle chilies for about 1-2 minutes, until fragrant but not burned.

Move the toasted chilies to a blender. Add 2 cups of beef stock and let them soak for 10 minutes to soften.

Add the quartered onion, peeled garlic cloves, and apple cider vinegar to the blender with the chilies. Blend everything until it becomes a smooth paste.

Pour the chili paste over the beef in the slow cooker. Add the remaining 2 cups of beef stock, bay leaves, oregano, cumin, smoked paprika, cinnamon stick, salt, and pepper.

Cover the slow cooker and cook on low for 8-10 hours until the beef is tender and easily shreds.

Once cooked, take the beef out and shred it using two forks. Discard any large fat pieces. Strain the cooking liquid and keep it warm.

Heat a skillet or griddle over medium heat. Toast each corn tortilla until warm and flexible.

Dip each tortilla into the reserved warm cooking liquid to lightly coat. Fill with the shredded beef and top with chopped cilantro and onion.

Fold the tacos and serve immediately with lime wedges on the side for squeezing over the tacos.