Heat a large skillet over medium high heat and add sliced onions. Let onions cook till they turn a golden brown color; stirring occasionally. Remove from skillet and set aside.
In the same skillet add the shredded chicken, 1 1/2 cups enchilada sauce, black beans, pinto beans, and cilantro. Season with salt and pepper. Let simmer over medium-high heat, stirring occasionally for 3-5 minutes.
In an 8"x8" casserole dish tear up 3-4 of the corn tortillas into 1" pieces, and layer them on the bottom of the dish. Spoon half of the chicken filling on top of the torn tortilla pieces.
Layer the caramelized onions on top of the chicken filling, then lay the avocado slices on top of the onions. Spoon the remaining chicken filling over the avocados.
Tear up the remaining 3-4 corn tortillas to make a top layer over the chicken filling. Drizzle the remaining 1/2 cup of enchilada sauce evenly over the top layer. Cover with foil and bake for 20 minutes.
Remove foil and top with the shredded cheddar cheese, and black olives. Bake for an additional 5 minutes, or until cheese is melted and bubbly. Let sit for a few minutes before slicing.