Preheat oven to 375°.
Arrange half of tortilla wedges on a large baking sheet, overlapping some if necessary. Bake at 375° for 10 minutes or until crisp. Remove the tortilla wedges from pan. Repeat procedure with remaining tortilla wedges.
Combine salsa, green onions, and 1/2 cup sour cream, stirring with a whisk. Place half of the tortilla wedges in an 11 x 7-inch baking dish coated with cooking spray. Top evenly with 1 cup of chicken, about 1 cup of salsa mixture, and 1/2 cup of cheese. Repeat layers with the remaining tortilla wedges, 1 cup chicken, 1 cup salsa mixture, and 1/2 cup cheese (dish will be very full). Cover with foil coated with cooking spray. Refrigerate for 8 hours or overnight.
Preheat oven to 350°.
Remove casserole from refrigerator; let stand at room temperature 15 minutes. Bake casserole, covered, at 350° for 30 minutes. Uncover and bake an additional 20 minutes or until bubbly around the edges and cheese begins to brown.