Preheat the oven to 350°F. Coat an 8- or 9-inch square baking pan or 2-quart baking dish with cooking spray.
Spread 1/4 cup of enchilada sauce over the bottom of the baking dish. Arrange 4 tortilla halves over the sauce to cover the bottom of the pan.
Layer 1/3 of the shredded chicken, 1/3 of the pinto beans, 1/2 cup of Monterey Jack cheese, and 1/3 cup of enchilada sauce over the tortillas.
Add a second layer of tortilla halves and repeat the layering with 1/3 of the chicken, beans, cheese, and sauce.
Repeat the layering process one more time with the remaining chicken, beans, cheese, and sauce. Top with the remaining tortilla halves and spread the last of the enchilada sauce over the top.
Sprinkle the remaining cheese over the sauce-covered tortillas.
Cover the dish with foil that's been lightly sprayed with cooking spray and bake for 30 minutes. Remove the foil and bake for an additional 5-10 minutes, or until the cheese is melted and slightly browned.
Allow the casserole to sit for 5 minutes before serving to help it firm up. Serve with optional garnishes, such as olives, sour cream, red onions, cilantro, green onions, avocado, or tomato if desired.