Preheat oven to 325 degrees
In a large saucepan, cook onion in melted margarine until tender, about 5 minutes.
Add soups, ROTEL tomatoes and chicken, stirring until well blended.
In a 13x9 inch baking pan, alternately layer tortillas, soup mixture, and cheese, repeating for 3 layers.
Bake 40 minutes or until hot and bubbling.