Place the beef in a large pot with the quartered onion, garlic cloves, bay leaves, a generous pinch of salt, and enough water to cover.

Simmer over medium heat until the meat is very tender, approximately 1 hour in a regular pot or 40 minutes in a pressure cooker.

Remove the meat from the pot and let it cool briefly. Reserve some cooking liquid (about 1–2 cups). Shred the beef finely with two forks or by hand.

In a blender, combine the roasted tomatoes, rehydrated guajillo and ancho chiles, half onion, garlic cloves, cumin, and a splash of reserved cooking liquid. Blend until silky.

Taste and adjust salt.

Strain the sauce through a fine sieve for a smooth texture.

Heat a large skillet with a little oil or lard. Pour in the strained sauce and fry for 3–5 minutes to remove raw edge and concentrate flavors. Season with salt.

Add the shredded beef to the skillet and stir until well-coated.

Simmer on medium-low for about 8–12 minutes so the meat soaks up the sauce. Taste and correct seasoning.

Serve hot.