Place dried chiles into a saucepan and cover with water. Bring to a boil, reduce heat, and simmer until chiles are softened, 15 to 20 minutes.

Pour chile mixture into a blender. Add garlic, onion slice, pepper, oregano, cumin, and cinnamon. Blend until smooth.

Layer chopped onions in the bottom of a slow cooker. Place chuck roast over onions and add bay leaves. Pour chile sauce on top.

Cook on High until beef is tender enough to shred with a fork, about 5 hours.

Remove beef from sauce. Continue cooking sauce until reduced, 35 to 45 minutes. Return beef to sauce and season with salt.