Preheat oven to 350 degrees F.
Lightly spray a 9 x 13 inch baking dish with non stick cooking spray.
In a saucepan combine soup, milk, sour cream, chiles, garlic salt, salt, oregano, and chili powder. Cook over low heat for 5 minutes.
Pour a thin layer of sauce from above to cover the bottom of pan.
Then add the shredded chicken to the remaining soup/sauce mixture.
Layer 5 tortillas on top of the very thin layer in the pan. The tortillas will overlap slightly. You will need to cut some of them in half and piece them to cover up the entire bottom of the pan.
Layer half of the soup/sauce mixture on the tortillas, then half of the cheese, then sprinkle on diced onions if desired.
Then repeat those layers again starting with tortillas, soup/sauce mixture, optional onions, and cheese on top.
Cover with foil and bake for 45-55 minutes.