CUT zucchini lengthwise in half; scoop out centers with spoon, leaving 1/4-inch-thick shells. Chop removed zucchini; place in small bowl. Add chicken, tomatoes and onions; mix lightly.
SPOON into zucchini shells; place on microwaveable plate. Microwave on HIGH 1 min.
TOP with cheese. Microwave 30 sec. or until cheese is melted.