Step 1: Preheat the Oven

1. Preheat your oven to 350°F (175°C).
2. Lightly grease an 11×9-inch casserole dish to prevent sticking.

Step 2: Prepare the Chicken Mixture

1. In a skillet over medium-high heat, combine shredded chicken, diced onion, taco seasoning, and ¼ cup water.
2. Cook for 5 minutes, stirring occasionally, until the flavors are well combined.
3. Set the mixture aside to cool slightly.

Step 3: Assemble the Casserole

1. Spread ½ cup of enchilada sauce evenly on the bottom of the casserole dish.
2. Layer 2 tortillas to cover the bottom of the dish.
3. Spread ⅓ of the chicken mixture over the tortillas, followed by ½ cup enchilada sauce and 1 cup shredded cheese.

Step 4: Repeat Layers

1. Repeat the layering process two more times: tortillas, chicken mixture, enchilada sauce, and cheese.
2. For the final layer, place the remaining 2 tortillas on top, pour the remaining enchilada sauce, and sprinkle with the remaining cheese.

Step 5: Bake the Casserole

1. Cover the dish tightly with aluminum foil and bake for 30 minutes.
2. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.

Step 6: Rest and Serve

1. Let the casserole rest for 5–10 minutes to set the layers.
2. Garnish with optional toppings like cilantro, sour cream, diced tomatoes, or avocado slices.