Season the chicken: Toss cubed chicken with taco seasoning until coated.
Layer in crockpot: Pour half of the enchilada sauce into the slow cooker. Add half the chicken, beans, onions, green chiles, tortillas, and cheese. Repeat the layers, ending with cheese.
Cook until tender: Cover and cook on low for 6 hours or high for 3 hours, until chicken is fully cooked and tender.
Stir and finish: About 15 minutes before serving, gently stir the casserole, then sprinkle with cilantro.
Optional toppings: Garnish with sour cream, avocado slices, extra cheese, salsa, or lime juice.