In a large saucepan, melt the butter over medium heat. Whisk in the cornstarch (or flour) and cook until golden brown. Slowly whisk in the chicken stock and cook until it begins to thicken. Stir in the milk, cumin, cayenne pepper, onion powder, chopped chiles and salt. Continue cooking until thickened. Remove from the heat and stir in the sour cream.

In a 7-qt slow cooker, line the bottom of the slow cooker insert with a layer of tortillas so that it is completely covered. Spoon 1/4 of the sauce over the tortillas. Layer 1/3 of the chicken on the sauce and sprinkle 1/4 of the cheese over the chicken. Repeat with remaining ingredients with the final layer being tortillas, sauce and cheese.

Place the lid on the slow cooker and turn on high. Cook for 3-4 hours. (Alternatively, you can cook the casserole on low for 6 hours). Serve with chopped avocado, sliced jalapeños, chopped cilantro and sour cream, if desired.