How to Make It
Place a rack 4 inches from heat source and preheat broiler to high. Place chiles in a single layer on a baking sheet. Broil until skin is blackened, 6 to 7 minutes, turning occasionally. Place in a large bowl; seal with plastic wrap. Let stand until skin has softened, about 7 minutes. Peel, seed and roughly chop chiles. Use immediately or refrigerate overnight in a ziplock bag.
Season pork with salt and pepper. Place in a slow cooker. Add chiles, enchilada sauce and tomatillos; turn to coat.
Cover and cook on low until meat is fork-tender, 6 to 7 hours. Remove pork to a baking dish and shred with two forks. Tilt slow cooker and spoon off some fat. Place pork back in slow cooker, cover and rewarm on high heat. Season with salt and pepper; serve with tortillas, if desired.