Heat oil in a small frying pan. Saute onion until tender. Add green chilies, olives, and chicken, and heat through. Remove from heat and moisten with a little enchilada sauce.
Coat an 8" baking pan with cooking spray, and pour a thin layer of enchilada sauce on the bottom. Line bottom with two tortillas, tearing to fit. Sprinkle with 1/3 chicken mixture and 1/4 of cheese. Drizzle with 1/4 of remaining sauce.
Repeat layers two more times. Top with remaining 2 tortillas, cheese, and sauce.
Cover pan with foil and bake at 350F until hot through - about 10 minutes. Remove foil and bake 10 more minutes, until cheese is toasty. Let set 10 minutes, and serve garnished with sour cream and avocado as desired.