Boil chicken and de-bone. Shred chicken and place in pan. Add onion, sour cream, cream of chicken soup and tomatoes. Stir together. Add chili powder and cumin to taste.
In casserole dish, tear tortillas into 4 pieces and place on bottom of pan. Layer chicken mixture, covering all tortillas and then add layer of shredded cheese. Repeat process until casserole dish is full.
Place in a 350°F oven for 30 minutes or until cheese is bubbling.