Make chicken tortilla casserole by layering tortillas, chicken and chicken gravy in a casserole dish and baking it. Start by cooking four boneless, skinless chicken breasts in boiling water for 30 minutes and then shredding the meat. Save 3 1/3 cups of the chicken cooking water.
To make the chicken gravy, mix 6 tablespoons of all-purpose flour into 6 tablespoons of melted butter over medium heat until it browns, and then add 1 cup of the reserved cooking water and 1 cup of milk. Once the gravy thickens, season it with salt and pepper to taste, and remove it from heat. In another pan, add the remaining cooking water, the prepared chicken gravy, 1 cup of canned green chiles, one finely-chopped onion and one minced garlic clove. Stir in 1 cup of sour cream, 1/2 teaspoon of cumin, 1/2 teaspoon of salt and 1/4 teaspoon of pepper, and bring everything to a boil before removing it from the heat.
Layer 1 cup of the sauce into the bottom of a casserole dish followed by a layer of six tortillas, 1 cup of shredded chicken and 1/2 cup of shredded cheddar cheese. Add three more layers of each ingredient, topping it with the last of the shredded cheddar cheese. Bake the casserole uncovered at 350 degrees Fahrenheit for 30 minutes.