Prepare stuffing as directed on package.
Meanwhile, melt butter in large nonstick skillet on medium heat. Add chicken; sprinkle with pepper. Cook 5 min. on each side or until chicken is done (165ºF); transfer to 13x9-inch baking dish sprayed with cooking spray.
Cook and stir beans, mushrooms and onions in same skillet 6 to 8 min. or until beans and onions are crisp-tender. Sprinkle with flour; cook and stir 1 min. Stir in broth; simmer 3 min. or until thickened, stirring frequently. Add cream cheese; cook and stir 2 to 3 min. or until melted. Spoon over chicken; top with stuffing. Refrigerate up to 24 hours.
Heat oven to 400ºF. Bake casserole (uncovered) 30 min. or until heated through.