Dice the onions, rough chop the garlic, remove/discard the stems from the dried chiles and shake out the seeds. Discard the seeds and cut the chiles into 2-3 pieces each (will get pureed once cooked, so this is just to fit into slow cooker).

SEAR BEEF. Preheat a frying pan or skillet on high heat. Season your chuck roast with salt, and pepper to coat all sides. Drizzle the olive oil in the hot pan and add the beef and sear for 2 minutes on each side to brown evenly. On the final side, pour 1 cup of the beef stock in the pan to scrape up the browned bits from the bottom of the pan. Transfer the beef to the slow cooker and pour in the liquid from the pan.

SLOW COOKER. To the slow cooker with the beef, add the onions, garlic, cilantro leaves, tomato sauce, chiles, cinnamon, cumin, paprika, salt, pepper, oregano, thyme, and bay leaves. Stir to combine. Then pour in the rest of the beef stock until it reaches the top of the beef. Set the slow cooker to Low 8 or 10 hours, depending on your schedule.

SHRED BEEF. When the cooking time is done, transfer the beef using tongs to a platter or cutting board. Shred the beef with forks and discard any string that may have been tied around the roast. Transfer the beef to a bowl and set aside.

BLEND SAUCE. Fish out the bay leaves and discard. Using a slotted spoon, scoop out all of the onions and ancho peppers and transfer to a blender. (alternatively, you could strain the liquid into another bowl, add then add onions and peppers to the blender.) Blend the onions and the peppers until smooth. If needed, add ½ cup of the cooking liquid to the blender to help smooth it out. Once smooth, pour it back into the liquid in the slow cooker and stir to combine. Finally, pour ½ cup of this sauce over the beef and gently stir to mix through.