Preheat oven to 300 degrees F (150 degrees C). Prepare rice according to package directions.
In a medium bowl, toss together the cooked chicken, prepared rice, soup, pimientos, and onion. Transfer to a lightly greased 9x13 inch baking dish. Stir in 1/2 cup of the cheese, then sprinkle the other 1/2 cup of cheese on top.