Toast the dried guajillo chilies in a dry skillet over medium heat for 2 minutes until fragrant, then soak in hot water for 10 minutes to soften.
Remove stems and seeds from the soaked chilies and blend them with the beef bone broth, garlic, cumin, oregano, sea salt, and black pepper until completely smooth.
Heat avocado oil in a pressure cooker or heavy pot and sear the beef chuck roast on all sides until a deep brown crust forms.
Pour the blended chili sauce over the beef, cover, and cook under high pressure for 45 minutes or simmer on low for 6 hours until the meat is fork-tender.
Remove the beef from the liquid, shred it thoroughly with two forks, and return it to the pot to absorb the flavorful juices.
Briefly dip each corn tortilla into the top layer of the warm cooking liquid and toast in a hot skillet for 30 seconds per side.
Fill the warm tortillas with the shredded beef, top with diced white onion and fresh cilantro, and serve with a small side of the remaining broth for dipping.