Preheat oven to 375F. Arrange half of tortilla wedges on a large baking sheet, overlapping some if necessary. Bake at 375F 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 350F. Remove casserole from refrigerator; let stand at room temperature 15 minutes. Bake casserole, covered, at 350F for 30 minutes. Uncover and bake an additional 20 minutes or until bubbly around the edges and cheese begins to brown.