01 - Set oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - In a medium skillet over medium heat, cook diced onion until softened, about 3 minutes. Add minced garlic and cook for 1 more minute. Stir in black beans and corn, then remove from heat.
03 - In a large bowl, mix shredded chicken, sautéed vegetables, and ½ cup enchilada sauce until well combined.
04 - Pour ½ cup enchilada sauce evenly over the bottom of the prepared baking dish. Arrange 4 tortillas over the sauce, overlapping as needed.
05 - Spread half of the chicken mixture over the tortilla layer. Sprinkle with ¾ cup shredded cheddar cheese.
06 - Add another layer of 4 tortillas, remaining chicken mixture, and ¾ cup shredded cheddar cheese.
07 - Place the last 4 tortillas on top. Pour the remaining enchilada sauce evenly over them. Sprinkle with the rest of the cheddar and Monterey Jack cheese.
08 - Cover the dish with foil and bake for 20 minutes.
09 - Remove foil and continue baking for 10 to 15 minutes until the casserole is bubbly and golden brown.
10 - Allow to rest for 10 minutes before slicing. Garnish with chopped cilantro, sliced avocado, and lime wedges. Serve with additional sour cream.