Place cubed chuck roast in slow cooker. Set aside.
In a food processor or blender, blend the remaining ingredients listed under "For The Barbacoa" until smooth. Pour sauce over the meat, toss to combine, cover, and cook on low for 6-8 hours or high for 3-4 hours or until the beef falls apart easily when shredded with a fork.
Set the slow cooker to the "warm" setting. Using 2 forks, shred the meat inside of the crockpot. Toss. Allow the newly shredded beef to sit with the juices for an additional 10 minutes while you prep the fixings.
Optional: Spray a skillet with cooking spray over medium-high heat. Cook corn tortillas 1-2 minutes per side to char. This also helps the tortillas to remain intact. You could also double-up the corn tortillas to prevent tearing.
Using tongs, serve the meat on corn tortillas and top with cilantro & onion. Enjoy!