Cheese-Vegetable Casserole

* 1 onion, chopped
* 1 (10 3/4 oz.) can cream of mushroom soup
* 1 (10 3/4 oz.) can cream of chicken soup
* 1 (8 oz.) jar Cheez Whiz
* 2/3 cup milk
* 1 cup Minute rice
* 1/2 cup water
* 2 pkg. frozen broccoli, cauliflower and carrots