01 - Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish evenly.
02 - In a medium skillet over medium heat, sauté diced onion until tender, about 3 minutes. Add minced garlic and cook for 1 minute more. Stir in black beans and corn, then remove from heat.
03 - In a large bowl, mix shredded chicken, sautéed vegetables, and 1/2 cup enchilada sauce until well combined.
04 - Pour 1/2 cup enchilada sauce into the baking dish and spread it evenly over the bottom.
05 - Arrange 4 tortillas over the sauce, overlapping as necessary. Spread half of the chicken mixture evenly on top, then sprinkle with 3/4 cup shredded cheddar cheese.
06 - Repeat layering with 4 more tortillas, remaining chicken mixture, and another 3/4 cup cheddar cheese.
07 - Finish with remaining 4 tortillas, pour over remaining enchilada sauce, and top with the rest of cheddar and Monterey Jack cheeses.
08 - Cover the dish with foil and bake for 20 minutes.
09 - Remove foil and bake an additional 10 to 15 minutes until bubbly and golden brown on top.
10 - Allow casserole to rest for 10 minutes before slicing. Garnish with chopped cilantro, sliced avocado, and lime wedges. Serve with extra sour cream.