01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish evenly.
02 - In a medium skillet over medium heat, sauté onion until soft, about 3 minutes. Add garlic and cook 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 into prepared baking dish, spreading evenly. Layer 4 tortillas over sauce, overlapping as needed.
05 - Top tortillas with half the chicken mixture, then sprinkle with ¾ cup shredded cheddar cheese.
06 - Add another layer of tortillas, remaining chicken mixture, and ¾ cup cheddar cheese.
07 - Top with remaining tortillas, spread remaining enchilada sauce, and sprinkle the rest of cheddar and Monterey Jack cheeses.
08 - Cover dish with foil and bake for 20 minutes.
09 - Remove foil and bake an additional 10 to 15 minutes, until bubbly and golden.
10 - Let casserole rest 10 minutes before slicing. Garnish with cilantro, avocado slices, and lime wedges. Serve with sour cream.