Preheat Oven: Preheat your oven to 375°F (190°C) and grease a 9×13-inch baking dish with cooking spray.
Prepare Chicken Mixture: In a large mixing bowl, combine the cooked shredded chicken, red enchilada sauce, diced green chilies, diced onion, diced bell pepper, ground cumin, garlic powder, salt, and pepper. Mix until all the ingredients are well combined.
Layer Tortillas: Arrange half of the corn tortilla quarters evenly in the bottom of the prepared baking dish.
Add Chicken Mixture: Spoon half of the chicken mixture over the layer of tortillas, spreading it out evenly.
Add Cheese: Sprinkle half of the shredded Monterey Jack and cheddar cheeses over the chicken mixture.
Repeat Layers: Repeat the layers with the remaining tortilla quarters, chicken mixture, and shredded cheeses.
Cover and Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
Uncover and Bake: Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
Garnish and Serve: Garnish the casserole with chopped fresh cilantro and serve with sliced avocado and sour cream, if desired.