Boil and shred chicken breasts. Spray 13x9x2 inch baking dish with Pam.
Rip tortillas into pieces. Combine chicken soup, sour cream and green chilies. Combine shredded chicken, 1/2 of the grated cheese and the chopped onions in a large bowl. Using 1/2 of the ingredients, begin layering in the baking dish. Start with tortilla pieces, followed by the chicken mixture. Add a layer of the soup mixture, then a layer of cheese. Repeat the layers. Bake, uncovered, at 350 degrees for 45 minutes. Serves 6.