Grease a 9 x 13" casserole dish with cooking spray. Spread a layer of enchilada sauce on the bottom of the dish (this keeps the tortillas from sticking).
Top with two tortillas. Cut the overlapping portions off and layer along the sides as shown in the assembly picture.
Sprinkle with 1/3 of the black beans, 1/3 of the chicken, and then 1/4 of the cheese. Repeat with a second layer in the same order. Before sprinkling cheese on the third layer, pour the remaining enchilada sauce evenly over the casserole and spread around. Add remaining amount of cheese (double the amount used in the first two layers) and sprinkle sliced olives on top.
Cover dish with foil and place in preheated oven. Let bake for 20 minutes covered. Uncover and bake another 10 - 15 minutes until cheese is melted.
Remove from oven and let sit for 10 minutes. Sprinkle green onion, avocado,* and cilantro on top. Serve with sour cream.
Notes
*If the entire casserole won't be eaten right away, put avocado on individual servings so that it doesn't turn brown in the fridge. ** This recipe was inspired by and adapted from Ali's Chicken Enchilada Casserole recipe over on Gimme Some Oven.