Blend the dried ancho chiles (seeded), garlic, lime juice, apple cider vinegar, cumin, chili powder, oregano, salt, and black pepper into a smooth paste, adding olive oil so it blends easily. Stop and scrape as needed until no big chile pieces remain.

Place the beef chuck roast in a crockpot and rub it all over with the chile paste, pressing to coat the surface. Make sure every side is evenly covered with the paste.

Add the onion halves and bay leaves to the crockpot around the roast. Pour any remaining chile paste or liquid over the meat so it sits partially submerged.

Cover and cook on low for 8 hours, until the meat is extremely tender and easily shreds. Visual cue: the roast should fall apart when pulled with a fork.

Remove the beef and shred it using two forks, then discard solids (onion and bay leaves) from the crockpot. Return shredded meat to the sauce so it stays moist.

Let the shredded barbacoa rest, covered, for 10 minutes so the juices settle back into the meat. Visual cue: the sauce should look glossy and slightly thickened.

Warm the small corn tortillas until pliable, soft, and lightly steamed. Visual cue: edges should flex without cracking.

Fill each tortilla with shredded barbacoa and pile it slightly higher in the center. Top with diced onion and fresh cilantro, then serve with lime wedges for squeezing.