Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
Sauté diced onion in olive oil over medium heat for 5 minutes until translucent.
In a large mixing bowl, combine the shredded chicken, cooked rice, black beans, corn, and sautéed onion.
Pour the enchilada sauce over the mixture and add chili powder, cumin, salt, and pepper.
Fold the ingredients together until the rice and chicken are uniformly coated with sauce.
Transfer the mixture to the prepared baking dish and spread into an even layer.
Sprinkle the shredded Mexican cheese blend evenly over the top.
Bake for 25 minutes until the cheese is melted and the edges are bubbling.
Allow to rest for 5 minutes before serving.