Preheat your oven to 350°F (175°C). Lightly spray the bottom of a 9 x 13-inch baking dish with non-stick spray to prevent sticking.
Layer 6 tortillas evenly at the bottom of the prepared baking dish.
In a mixing bowl, combine the cream of chicken soup, milk, sour cream, chopped green chilis, salt, pepper, garlic powder, and onion powder. Stir until well mixed.
Spread half of the shredded chicken evenly over the layer of tortillas.
Pour half of the creamy mixture over the chicken, making sure it’s distributed evenly.
Sprinkle half of the shredded Mexican Fiesta cheese over the top.
Repeat the layering process: add another 6 tortillas, the remaining chicken, the remaining creamy mixture, and the remaining cheese.
Bake in the preheated oven for 40 minutes, or until the casserole is bubbly and the cheese is melted and slightly golden.
Once baked, let the casserole rest for a few minutes before serving. Garnish with chopped green onions if desired.