01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - In a skillet over medium heat, sauté chopped onion until softened, about 3 minutes. Add ground cumin, chili powder, garlic powder, and salt; cook for 1 more minute. Remove from heat.
03 - In a large bowl, mix shredded chicken, rinsed black beans, thawed corn, sautéed onions, and ½ cup enchilada sauce until well combined.
04 - Spread ½ cup enchilada sauce evenly on the bottom of the prepared baking dish. Layer 5 tortillas over the sauce, tearing to cover completely.
05 - Spread half of the chicken mixture over the tortillas and sprinkle with one third of the shredded cheddar cheese.
06 - Drizzle with ½ cup enchilada sauce.
07 - Repeat layering with remaining tortillas, remaining chicken mixture, another one third of the cheese, and ½ cup sauce.
08 - Finish with remaining cheese and any leftover enchilada sauce evenly spread on top.
09 - Cover casserole with aluminum foil and bake for 20 minutes.
10 - Remove foil and bake an additional 10 to 15 minutes until cheese is bubbling and golden brown.
11 - Allow casserole to rest for 10 minutes before serving. Garnish with cilantro and jalapeño slices if desired. Serve with sour cream on the side.