Preheat oven to 375 degrees. Spray a 13 x 9 inch pan with non-stick cooking spray.
In the prepared pan, layer half of the chicken, enchilada sauce, sour cream, tortillas, chilis, tomatoes and cheese. Repeat layers with remaining ingredients.
Cover pan with aluminum foil and bake for 35-40 minutes. Remove foil and bake for 10 more minutes or until it starts to bubble.
Let stand for 10 minutes before serving. Top with additional tomatoes and diced avocado.