Preheat oven to 350F. Spray a 9x13" baking dish with cooking spray.
Place 6 tortillas in the bottom, overlapping as needed. Top with one jar of green enchilada sauce, spreading out to the edges. Place on top 1/2 of the chicken, 1/3 of the cheese and 1/2 of the onion. Layer on top 6 more tortillas, followed by the red enchilada sauce and then the remaining chicken, 1/3 of the cheese and remaining 1/2 of the onion. Top again with 6 more tortillas, the other jar of green enchilada sauce and the remaining cheese.
Bake in the oven for 30-40 min. until nicely browned on top and bubbling. Let stand for 5-8 min. Top with the shredded lettuce, black olives and tomatoes. Cut into squares and serve.
Nutrition calculation is an estimation only. If you need nutritional calculations for medical reasons, please use a source that you trust.