In the slow cooker pot, whisk together beer, apple cider vinegar, torn anchos, tomato paste, garlic, cumin, oregano, salt and cinnamon.
Arrange sliced onion rounds in a single layer in the pot. Place beef chunks over the onions. The onions will protect the beef from the heat at the bottom of the pot. If needed, gently push down the beef so it is covered with the sauce.
Cover slow cooker with lid and cook for 4-5 hours on low heat. Cook until the beef is browned and tender.
Use kitchen tongs to transfer cooked beef to a large bowl. Set aside.
Remove onion rings from slow cooker pot and discard. Take remaining cooking sauce and transfer to a blender. Puree until smooth. Alternatively, use an immersion blender. Reserve sauce.
Use two forks to shred cooked beef. Pour reserved sauce over shredded beef. Toss to coat beef in sauce. Season with salt as necessary.