HEAT oven to 400ºF. Microwave potatoes and water in 2-qt. microwaveable ovenproof casserole on HIGH 10 to 12 min. or until potatoes are tender. Let stand 5 min. Meanwhile, heat dressing in nonstick skillet on medium-high heat. Add meat and onions; cook and stir until meat is browned. Stir in corn and tomatoes.

DRAIN potatoes; return to dish. Flatten with bottom of small cup or bowl; top with meat mixture and cheese. Cover.

BAKE 25 min. or until casserole is heated through and top is golden brown, removing lid after 20 min.