Toast the dried guajillo and ancho chiles in a dry pan, stirring constantly, until fragrant, about 30–60 seconds, with a slight darkening on the edges. This should not burn.

Blend the toasted chiles with diced tomatoes, chopped onion, garlic cloves, chipotle in adobo, beef broth, apple cider vinegar, cumin, oregano, cinnamon, and salt and black pepper until smooth, about 1–2 minutes. Blend until the sauce looks fully red and uniform.

Place the beef chuck roast chunks in your crockpot and pour the chile sauce over the top. Make sure the beef is mostly covered.

Cook on low for 8 hours. The beef should be fork-tender and the sauce should look deeply red.

Shred the beef using two forks and reserve the consommé separately. Keep the top layer of red consommé fat for dipping.

Heat a cast iron skillet over medium-high heat until hot, about 2–3 minutes, indicated by quick sizzling when a tortilla touches the surface. Keep it ready for fast frying.

Dip corn tortillas in the reserved red consommé fat (from the top layer), coating quickly and evenly. The tortillas should look glossy and lightly stained red.

Cook the dipped tortillas in the hot skillet for 1 minute until beginning to crisp. You should see small golden spots.

Add shredded Oaxacan or mozzarella cheese and birria beef to one half of the tortilla. The cheese should sit directly on the tortilla where it will melt.

Fold the tortilla in half and press gently, cooking 2–3 minutes per side until crispy and the cheese is melted. The tacos should be browned at the edges and cohesive at the fold.

Serve immediately with a cup of consommé for dipping, diced white onion, and fresh cilantro. The consommé should steam when poured.