Place the 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 sauce 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 the chile sauce on top. Cook on High until beef is tender enough to shred with a fork, about 5 hours.
Remove beef from the sauce. Continue cooking sauce until is reduced, 35 to 45 minutes. Add beef back in and season with salt.
