Blister the dried guajillo and cascabel chiles on a comal over medium heat, then submerge chiles in boiling water and let stand for 10 minutes to rehydrate.
Season the beef cheek with salt and pepper evenly on both sides according to your taste and preference; set aside.
In a blender or a food processor, combine the chiles with the garlic, oregano, cumin, and 2–3 tablespoons of the water. Purée until smooth, adding water as needed to give it a thick but pourable consistency. Transfer to a large pot or Dutch oven.
Add the rest of the water, onion, and bay leaf to the chile mixture and season well with salt and pepper. Stir to combine and bring to a rolling simmer over medium heat.
Add the beef cheek to the pot, reduce heat to low, and cover. Cook until meat is fork-tender, 1½–2 hours.
When the meat is beginning to fall apart (but not mushy or dissolving), remove with a slotted spoon, and place in a clean bowl. Using two forks, shred into bite-sized pieces. Meanwhile, increase the heat to medium-high and bring the remaining liquid to a light boil. Reduce until consistency has thickened slightly, and season to taste; add some of this braising liquid to the cheek meat just before serving for extra juiciness, or ladle into small bowls so guests can serve themselves.
Serve immediately with warm tortillas, salsa, and cilantro.
Combine all of the ingredients for the salsa in a blender or food processor and puree.