Place the chicken breasts in the slow cooker. Add the green chiles, chili powder, cumin, garlic powder, kosher salt, chopped onion, and approximately 1 cup of the enchilada sauce, keeping the remaining sauce aside for layering.

Cover and cook until the chicken reaches 165°F internally and shreds easily, about 3–4 hours on HIGH or 6–7 hours on LOW. Avoid cooking the chicken substantially longer than needed, since it can become dry even when surrounded by sauce.

Remove the cooked chicken from the slow cooker and shred it with two forks. Return the shredded chicken to the cooking sauce and stir until it is evenly coated.

Layer the tortilla strips or wedges in the slow cooker, followed by some of the shredded chicken mixture, remaining enchilada sauce, and about half of the shredded cheese. Repeat the layers as needed, finishing with enchilada sauce and cheese on top.

Cover and cook on HIGH for 20–30 minutes, or until the tortilla pieces are tender and the cheese is melted. Turn off the slow cooker and let the casserole stand for about 5 minutes before serving.

Spoon the enchiladas into bowls or onto plates and add your preferred toppings. Try sour cream, cilantro, green onions, avocado, or sliced jalapeños for a fresh and colorful finish.