Microwave potatoes and water in 2.3-L microwaveable ovenproof casserole dish 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. or until meat is done, stirring occasionally; stir in corn and tomatoes.