Heat one cup of water in the microwave or stovetop for 90 seconds or until almost boiling. Place the Guajillo chile in the water for 10 minutes to rehydrate.
Heat a griddle, comal, or large skillet over high heat and add the tomatillos, Serrano chile, onion, and garlic. Char them on all sides.
Place the charred veggies in a blender, add the rehydrated Guajillo, ½ a cup of the rehydrating water, cilantro, salt, and pepper. Blend until well combined.
Heat a large skillet over high heat and add the olive oil. Brown the pork tenderloins on all sides.
Place the browned tenderloins into the slow cooker, pour the sauce over them, and add the bay leaf. Place the lid on and cook on low for 8 hours, or on high for 4.
Remove the tenderloins from the slow cooker and shred them using two forks. They should be fork-tender. Try the sauce left in the slow cooker and add more salt if needed.
Return the shredded meat to the slow cooker and mix it with the sauce.