Preheat oven to 350 degrees. Coat a 9 x 9-inch square baking pan or 2-quart casserole with cooking spray; set aside.
In a large saucepan, cook rice according to package directions.
Meanwhile, in a microwave-safe bowl, add the onions and garlic and cook in the microwave for about 1 minute, to soften onions.
When rice is done, add it to a large mixing bowl. Mix in cooked onions and garlic, corn, diced chicken, tomatoes, tomato sauce, ½ cup of the cheese, chili powder, and cumin. Mix well. Transfer to the casserole dish. Sprinkle with the remaining ½ cup cheese. Bake, uncovered, for 20 to 25 minutes or until cheese is lightly browned and casserole is heated through.
Cool at least 10 minutes to set. Cut into 6 servings. If desired, top each with 1 tablespoon sour cream or yogurt, and 1 tablespoon salsa.