Chicken Tortilla Casserole

4 cups Boneless skinless chicken breasts, cooked and cut into bite size pieces
1 ( 14 oz.) can fat free evaporated milk
1 (6 oz.) can tomato paste
1 cup light sour cream
1 Onion, grated
1 teaspooon oil
2 Small cans green chili salsa
16 Large corn tortillas, cut in 1 inch squares
12 Ounces shredded low fat cheddar cheese
1/4 Cup sliced black olives
In a medium sauce pan sauté onion in oil..  Combine evaporated sour cream, tomato paste, and chili salsa, with onion.  Heat slowly until warm about 15 minutes.  Spray casserole dish with non-stick vegetable spray.  Layer half the tortillas, chicken, salsa mixture, and cheese in casserole dish.  Repeat layer.   Garnish with olives.  Cover and bake for 1 hour and 30 minutes in 350 degree oven