Preheat oven: Set to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
Cook aromatics: Heat olive oil in a skillet over medium heat. Sauté onion until soft and fragrant, about 3–4 minutes. Remove from heat.
Prepare filling: In a large bowl, combine shredded chicken, beans, corn, sautéed onion, and 1 cup of enchilada sauce. Stir until well coated.
Layer casserole:
Spread a thin layer of enchilada sauce on the bottom of the dish.
Place tortillas to cover the base (tearing if needed to fit).
Spread half the chicken mixture over tortillas.
Sprinkle with ½ of the cheese.
Repeat layers (tortillas, filling, cheese), finishing with tortillas on top.
Top with sauce & cheese: Pour remaining enchilada sauce evenly over the top layer. Sprinkle with remaining cheese.
Bake: Cover with foil and bake for 20 minutes. Remove foil and bake for another 10–15 minutes, until cheese is melted, bubbly, and golden.
Rest & garnish: Let sit for 5 minutes before cutting into squares. Top with cilantro, jalapeños, sour cream, or avocado if desired.