Preheat the oven to 350°F (175°C). In a bowl, mix the shredded chicken, half of the green enchilada sauce, garlic powder, cumin, salt, and pepper. Set aside. This flavorful mixture will form the heart of your casserole.
In a baking dish, layer the corn tortillas at the bottom. You may need to cut them to fit the dish, ensuring that the entire bottom is covered. This will serve as your base, which will soak up all the enchilada flavors as it bakes.
Spread half of the chicken mixture over the tortillas, followed by a layer of sour cream, and top with a generous amount of shredded cheddar and Monterey Jack cheese. Repeat the layers, finishing with a final layer of cheese. The more cheese, the better!
Bake the casserole for 30-35 minutes until the cheese is melted, bubbly, and golden brown. Once it's done, let it cool for a few minutes before serving. Garnish with fresh cilantro for a burst of color and freshness!