Start by melting butter in a large skillet over medium heat. Stir in the flour and cook for about a minute to form a smooth paste. This step creates a roux that thickens your sauce beautifully.
Gradually whisk in the chicken broth until the mixture is smooth and slightly thickened. Then stir in the chili powder, oregano, salt, and black pepper. You’ll notice the sauce taking on a warm, golden color with a savory aroma.
Place the chicken breasts directly into the sauce, cover the skillet, and let them simmer on low until cooked through — about 12 to 15 minutes. The chicken will absorb all the wonderful flavors of the sauce while staying tender and juicy.
Remove the cooked chicken, shred it into bite-sized pieces, and return it to the skillet. Stir in the sour cream until it blends smoothly into the sauce. Add the tortilla pieces, diced tomatoes, and black beans (if using). Everything should be evenly coated in the creamy mixture.
Sprinkle the shredded cheese evenly over the top, cover, and cook for another 5–8 minutes on low heat until the cheese melts and the sauce is bubbling. Once done, remove from heat and let it sit for a few minutes to thicken slightly before serving.