Preheat your oven to 350°F and spray a 9x13 inch baking dish with cooking spray. While the oven heats, finely chop the onion, mince the garlic cloves, and measure out all your spices (chili powder, cumin, oregano) into a small bowl for easy access. Drain the canned green chiles and black beans, then combine them in a bowl with the shredded chicken, salt, and black pepper—set this mixture aside. Having everything prepped and ready ensures you can build the sauce smoothly without interruption.

Heat the olive oil in a large pan over medium heat, then add the chopped onion with 1/2 tsp salt and cook for 5 minutes until softened. Stir in the measured spice mixture (chili powder, cumin, oregano) for about 30 seconds to bloom the spices and release their flavors. Add the flour and stir constantly for 1 minute to create a roux base, which will help thicken and bind the sauce. Gradually whisk in the chicken stock, stirring to dissolve any lumps, then add the tomato sauce and minced garlic. Bring the mixture to a gentle simmer and cook for 5 minutes, stirring occasionally—this allows the flavors to meld together beautifully.

Pour 1/2 cup of the sauce from Step 2 into the bottom of your prepared baking dish, spreading it evenly to prevent sticking. Pour another 1/2 cup of sauce into the chicken and black bean mixture from Step 1, stirring gently to combine. Dip 6 corn tortillas one at a time into the remaining sauce and arrange them in a single layer over the sauce in the baking dish, overlapping slightly if needed. I find that dipping each tortilla briefly (about 2-3 seconds per side) keeps them pliable without making them soggy.

Spread half of the sauced chicken mixture from Step 3 evenly over the tortilla layer, then sprinkle 1 cup of the shredded cheese over the chicken. Dip the remaining 6 tortillas in the remaining sauce and layer them on top, then spread the remaining chicken mixture over these tortillas. Pour any remaining sauce from Step 2 over the casserole, ensuring the top is covered. I like to reserve the cheese topping for the very end because it helps seal in the moisture and creates that beautiful bubbly finish.

Sprinkle the remaining 2 cups of shredded colby Jack cheese evenly over the top of the casserole, covering all the visible sauce and tortillas. Place the baking dish in your preheated 350°F oven and bake for 25 minutes, or until the cheese is melted, bubbly, and beginning to brown slightly at the edges. Let the casserole rest for 5 minutes after removing from the oven—this allows the layers to set slightly and makes serving cleaner.