Preheat the Oven:
Preheat the oven to 350°F (175°C). Spray a 9×13-inch baking dish with non-stick cooking spray and set aside.

Prepare the Chicken Mixture:
In a medium-sized bowl, combine the shredded chicken, 1 cup of shredded Monterey Jack cheese, garlic powder, salt, and black pepper. Stir well to combine and set aside.

Make the Sauce:
In a medium saucepan, melt 3 tablespoons of butter over medium heat. Once melted, add the 3 tablespoons of flour and cook for about 1 minute to create a roux.

Add Broth and Cream:
Slowly whisk in 2 cups of chicken broth, sour cream, cream cheese (if using), and the diced green chiles with their juice. Whisk until smooth, and bring the mixture to a simmer. Continue to whisk until the sauce thickens. Once thickened, remove from heat.

Assemble the Casserole:
Stack the tortillas and use a knife to cut them in half or quarters (depending on the size of your pan). Lay 8 halves of tortillas on the bottom of the prepared baking dish. Spread 1/3 of the chicken mixture over the tortillas, then pour 1/3 of the sauce on top.

Layer and Repeat:
Repeat the layering process two more times: tortillas, chicken, and sauce.

Top with Cheese:
After the final layer of sauce, sprinkle the remaining 1 cup of shredded Monterey Jack cheese on top.

Bake the Casserole:
Bake uncovered in the preheated oven for 30 minutes, or until bubbly and heated through. If desired, broil on low for a few minutes to slightly brown the top. Watch closely to avoid burning.

Serve:
Garnish with fresh chopped parsley or cilantro if desired and serve hot.