In a large bowl, combine salsa verde, sliced onion, minced garlic, ground cumin, chili powder, salt, and black pepper. Stir well to mix all ingredients.
Place the chicken breasts in a gallon-sized freezer-safe bag or airtight container.
Pour the salsa verde mixture over the chicken in the bag or container. Seal tightly, removing as much air as possible if using a bag.
If freezing, label the bag or container with the date and contents, then place in the freezer.
When ready to cook, thaw the freezer meal in the refrigerator overnight.
Transfer the contents of the bag or container to the crockpot.
Cook on low for 6 hours or until the chicken is cooked through and tender.
Once cooked, shred the chicken with two forks and stir to combine with the sauce.
Sprinkle chopped fresh cilantro over the shredded chicken before serving.