PREPARE Potatoes as directed on package, using Basic Directions, except do not add the Cheese Sauce.
MEANWHILE, heat oil in large skillet on medium-high heat. Add chicken; cook and stir 8 to 10 min. or until done. Stir in tomatoes, mushrooms and Cheese Sauce; cook 5 min. or until heated through, stirring occasionally.
SERVE chicken mixture over potatoes.