Preheat your oven to 350°F (175°C). This ensures your casserole cooks evenly and gets that lovely golden top.
In a mixing bowl, combine 1 cup of the red enchilada sauce with the taco seasoning. Stir until well mixed. This will be the flavorful base for your casserole.
Spread a thin layer of the sauce mixture on the bottom of your baking dish. Take 6 flour tortillas and layer them over the sauce, slightly overlapping them.
Spread half of the shredded cooked chicken over the tortillas, followed by one cup of the shredded Mexican cheese blend. Drizzle another cup of the enchilada sauce over the chicken and cheese.
Add another layer of tortillas, followed by the remaining chicken, another cup of cheese, and another cup of enchilada sauce. Top with the remaining tortillas.
Pour the last cup of enchilada sauce over the top layer of tortillas. Finally, sprinkle the remaining cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Once out of the oven, let the casserole sit for about 10 minutes. Top with chopped tomatoes, sliced avocado, crumbled Cotija cheese, and fresh cilantro before serving. Enjoy your delicious Chicken Tortilla Casserole!