Prepare chicken: Season chicken with salt, pepper, cumin, and chili powder.
Mix sauce: In a bowl, combine salsa verde, sour cream, onion, and garlic.
Layer in crockpot: Spread a small amount of salsa mixture on the bottom of the slow cooker. Place half of the tortillas on top, overlapping slightly. Spread half of the chicken over tortillas, then pour half of the salsa mixture and sprinkle with 1 cup cheese. Repeat layers with remaining tortillas, chicken, salsa, and cheese.
Slow cook: Cover and cook on low for 4–5 hours or high for 2–3 hours, until chicken is fully cooked and enchiladas are heated through.
Shred chicken: Remove chicken and shred with two forks, then return to crockpot and stir gently to mix with sauce and cheese.
Serve: Garnish with chopped cilantro, avocado slices, extra cheese, or lime wedges as desired.