Combine the first 9 ingredients in a large saucepan, stirring with a whisk. Bring to a boil, stirring constantly. Remove from heat.
Spread 1 cup soup mixture in a 13 x 9-inch baking dish coated with cooking spray. Arrange 6 tortillas over the soup mixture, and top with 1 cup chicken and 1/2 cup cheese. Repeat layers, ending with the cheese. Spread remaining soup mixture over cheese. Bake at 350° for 30 minutes or until bubbly.