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. Place In Preheated Oven Briefly, To Melt.