Preheat oven to 350F.
Toast chilies in a dry, cast-iron Dutch oven over med-high heat until softened and pliable. Remove seeds and stems from chilies and add to a high-speed blender along with cinnamon, smoked paprika, apple cider vinegar, garlic & beef stock. Blend on high speed until completely smooth.
Pat beef dry with paper towel & season all sides with salt. Raise heat to med-high and add 2 tbsp avocado oil to Dutch oven. Brown the beef in batches, if required.
Set a fine-mesh sieve over the Dutch oven and pour the chili-stock mixture through the sieve. Use a silicon spatula to work the sauce through the sieve – discard any of the pulp. Toss the beef to coat in the sauce, add the thyme. Bring sauce to a simmer, cover with lid and transfer to oven. Cook 2.5 to 3 hours or until meat is fall-apart tender.
Carefully transfer cooked beef to a bowl or large platter & shred it with two large forks – set aside.
Pour the rendered sauce and juices into a degreasing cup and remove excess grease. Taste for seasoning and adjust salt and pepper accordingly. If required, reduce sauce by heating it in the Dutch oven over med-low heat until desired consistency is reached. Pour degreased sauce over the shredded beef and toss to coat.
Serve Whole30 Beef Barbacoa over a bowl of cauliflower rice. Garnish with shredded cabbage, salsa of choice, avocado, fresh cilantro & lime wedges.