PLACE potatoes in 2-L microwaveable casserole dish. Add water. Microwave on HIGH 6 to 8 min. or until potatoes are tender; stir. Let stand 5 min. Drain potatoes; rinse with cold water. Drain again.
COMBINE dressing, bacon, cheese and onions in large bowl. Add potatoes and tomatoes; mix lightly.
SERVE immediately, or cover and refrigerate several hours or until chilled.