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 large nonstick skillet on medium-high heat. Add meat and onions; stir. Cook 10 min. until meat is browned, stirring occasionally. Stir in corn and tomatoes.
DRAIN potatoes; return to casserole. Flatten potatoes with bottom of small cup or bowl; top with meat mixture and cheese. Cover.
BAKE 25 min. or until ingredients are heated through and top is golden brown, uncovering for the last 5 min.