1. Preheat oven to 300 degrees F (150 degrees C.) and spray 9x13 inch casserole dish w/ nonstick spray.
2. Crush up tortilla chips into smaller pieces; tear tortillas into strips.
3. Combine the tomatoes, chicken soup, mushroom soup and onion. Pour half of mixture over tortillas. Sprinkle crushed chips on top.
4. Layer the chicken pieces, half of the cheese and the remaining soup mixture. Top with more crushed chips.
5. Bake at 300 degrees F (150 degrees C) for 20 minutes. Top with the remaining cheese and return to the oven until the cheese is melted.
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