Preheat Oven and Prepare Baking Dish:Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.Layer Tortillas and Chicken:Cut the tortillas into strips or quarters for easier layering. Place half of the tortillas in the bottom of the prepared dish. Top with the shredded chicken and half of the chopped onion.Add Cheese and Repeat Layers:Sprinkle one cup of Monterey Jack cheese over the chicken. Layer the remaining tortillas, onion, and another cup of Monterey Jack cheese.Mix Sour Cream and Enchilada Sauce:In a bowl, mix together the sour cream and enchilada sauce until well combined. Pour this mixture evenly over the casserole layers.Final Cheese Layer:Sprinkle the cheddar cheese on top for a gooey, golden crust.Bake:Bake in the preheated oven for 30 minutes, or until the casserole is bubbly and the cheese has melted.Garnish and Serve:Let the casserole stand for a few minutes after baking. Garnish with chopped cilantro and serve with sliced avocados on the side, if desired.Serving Suggestions:Serve this hearty casserole with a side of refried beans or a fresh green salad for a complete meal.Consider adding a dollop of guacamole or extra sour cream on top when serving for added creaminess and flavor.
Notes
For a spicier version, you can add chopped jalapeños to the chicken layer or use a hot variety of enchilada sauce.