Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease a 9x13 inch ceramic or glass baking dish.
In a medium mixing bowl, combine sour cream, condensed cream of chicken soup, diced green chilis, chicken broth, garlic powder, cumin, salt, and pepper; whisk until a homogenous sauce forms.
Spread 1/2 cup of the prepared sauce across the bottom of the baking dish to prevent sticking and provide moisture to the base layer.
Place 6 corn tortillas in a single layer over the sauce, overlapping slightly to cover the entire surface area.
Distribute 1.5 cups of shredded chicken and half of the diced onions evenly over the tortilla layer, followed by 3/4 cup of shredded Monterey Jack cheese.
Pour half of the remaining sauce over the chicken and cheese layer.
Repeat the layering process using the remaining 6 tortillas, the remaining 1.5 cups of chicken, the remaining onions, and the remaining sauce.
Top the final layer of sauce with the remaining 1.25 cups of Monterey Jack cheese.
Bake uncovered for 30 minutes or until the internal temperature reaches 165 degrees Fahrenheit and the cheese is bubbling and slightly golden.
Remove from oven and allow the casserole to rest for 10 minutes to ensure structural integrity when slicing. Garnish with cilantro before serving.