Preheat oven to 325°F. Spray 13x9" casserole dish with non-stick spray and set aside.
Melt butter in a saucepan over medium heat. Once melted, add the peppers and onions; sauté for 2-3 minutes or until the onions are soft and translucent .
Add chicken and mushroom soups, diced tomatoes, and chicken to the pan with peppers and onions. Gently mix together.
Add 1/3 of the tortilla to the bottom of the casserole dish. Follow with 1/3 of the soup mixture, then top with 1 C. of shredded cheese. Repeat with two additional layers (1/3 tortillas + 1/3 soup mixture each time, plus 1 C. shredded cheese).
Bake uncovered, for 30-45 minutes or until the casserole is bubbly and cheese is completely melted.
Allow to stand for 5-10 minutes to cool before serving.