Preheat oven to 350ºF, and prepare a large casserole dish or 9 x 13 inch pan by coating with nonstick cooking spray.

In a large sauce pan, combine milk, cream soup, and rice. Bring mixture to a boil. Reduce heat and add chicken, green beans, mushrooms, and pimentos. Stir to combine.

Remove from heat and pour into the prepared casserole dish. Spread shredded cheese on top of the mixture. Sprinkle the onion rings on top of the cheese. Bake casserole uncovered for about 20 minutes.

Remove from the oven and cover the casserole loosely with foil to prevent over-browning. Bake an additional 20 minutes.