PREHEAT oven to 350°F. Cook and stir onion in 1 Tbsp. of the butter in large skillet on medium heat until tender.
ADD rice, soup, broccoli and CHEEZ WHIZ; mix lightly. Spoon into 1-1/2-qt. casserole. Toss together remaining 1 Tbsp. butter, melted, and bread cubes; sprinkle over casserole.
BAKE 30 to 35 min. or until heated through.