In a gallon size zip-top bag, combine pork, salsa verde, onion, garlic, cumin, red pepper flakes and black pepper. Knead bag to combine. Label and freeze.
When ready to cook, thaw in refrigerator overnight.
Transfer contents of bag to a slow cooker. Cover and cook on low 6-8 hours.
Shred pork and mix with sauce in the cooker. Serve over corn or flour tortillas with desired toppings.