Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a skillet, heat the olive oil over medium heat. Add the diced onion and cook until softened, about 3-4 minutes.
In a large bowl, mix the shredded chicken, cooked onion, green enchilada sauce, diced green chiles, sour cream, cumin, garlic powder, salt, and pepper.
Layer half of the tortilla pieces in the bottom of the prepared baking dish. Spread half of the chicken mixture over the tortillas. Sprinkle with half of the cheese.
Repeat the layers with the remaining tortillas, chicken mixture, and cheese.
Bake for 25-30 minutes, until the cheese is melted and bubbly. Let it rest for 5 minutes before serving.
Garnish with fresh cilantro before serving.
Notes
For extra heat, add a diced jalapeño or a pinch of cayenne pepper to the chicken mixture.
Keyword enchilada casserole, green chile chicken enchilada casserole, Mexican casserole