Roast the poblano peppers by laying them directly on gas stove over flame. Use tongs to rotate peppers often until they are charred on all sides. (Note: This step can also be done on a grill or under a broiler in the oven. Whatever the method, don’t skip this step as it produces a ton of flavor!)

Transfer charred peppers into a bowl. Cover and let steam for 10 minutes. Hold each pepper under running water and peel. Dry peppers and remove stems and seeds. Place peppers in food processor.

Preheat broiler to high.

Using medium bowl, add tomatillos, garlic cloves, serrano peppers and vegetable oil; toss until well coated.

Transfer tomatillo mixture to a sheet pan lined with foil. Broil for 5 minutes, flip and continue broiling for 5 more minutes, or until tomatillos are charred. Transfer mixture (along with any liquid) into food processor; pulse several times until well chopped, but not entirely pureed.

Pour mixture into a slow cooker.

Trim any excess fat off pork shoulder. Cut pork into 2” pieces. Using a large bowl, add pork, chili powder, cumin and salt; toss until well combined. Transfer pork into slow cooker.

Add green chilies and onion to slow cooker; stir until well combined. Cover and cook on low for 8-10 hours, or until pork is very tender.

Using a slotted spoon, transfer pork to a cutting board and shred using 2 forks. Cover shredded pork and set aside.

Transfer any remaining solids from slow cooker into a blender or food processor. Add cilantro and ¼ cup of cooking liquid from the slow cooker. Pulse several times until mixture is smooth.