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; stir. Cook 4 to 5 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 after 20 min.