Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
In a small skillet, heat olive oil over medium heat. Add the diced onion and cook until soft and translucent, about 3–4 minutes. Remove from heat.
In the baking dish, combine the cooked chicken, uncooked rice, black beans, corn, diced tomatoes (with their juices), sautéed onion, chicken broth, and all the seasonings. Stir everything together until well mixed.
Cover the casserole tightly with foil and bake for 35 minutes, or until the rice is fully cooked and the liquid is absorbed. Remove the foil and sprinkle the shredded cheese evenly over the top.
Return the dish to the oven uncovered for an additional 5–10 minutes, or until the cheese is melted and bubbly.
Let the casserole rest for 5 minutes before serving. Top with fresh cilantro, avocado, or your favorite fixings.