Preheat the oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish.

In a large skillet, heat the olive oil over medium heat and sauté the diced onion for 3 to 4 minutes, until softened.

In a large bowl, combine the cooked shredded chicken, rice, black beans, corn, salsa, enchilada sauce, taco seasoning, sautéed onion, salt, and black pepper.

Stir in 1 cup of the shredded cheese and the sour cream until everything is evenly mixed.

Spread the mixture into the prepared baking dish and smooth the top.

Sprinkle the remaining 1 cup of cheese evenly over the casserole.

Bake uncovered for 25 to 30 minutes, until the casserole is hot and bubbly and the cheese is melted.

Remove from the oven and let it rest for 5 minutes. Garnish with chopped cilantro and any optional toppings before serving.