Preheat the oven to 400F. Spray a 9x11-inch baking pan with cooking spray.
Layer the casserole. In a large bowl, mix the tomatillo salsa verde with the sour cream. Spread 1 cup of the sauce on the bottom of the pan. Layer 6 of the small tortillas on the bottom of the pan. Spoon 1 ¼ cups of the sauce over the tortillas then layer ⅓ of the chicken, ⅓ of the butternut squash and ⅓ of the black beans. Top with ⅓ of the cheese and repeat the process 2 more times, but set aside the last layer of cheese.
Bake. Spray one side of a piece of aluminum foil with cooking spray and cover the pan then bake for 60-70 minutes or until the sauce starts to bubble. Remove the foil and top with the remaining cheese and bake until melted, about 10 more minutes.
Top and serve. Remove from the oven and top with sliced olives, green onions, and cilantro. Garnish with sour cream, avocado slices and pickled jalapeños. Optionally sprinkle Cotija cheese on top for a tart cheese burst of flavor.