REMOVE outer layer of leaves from Brussels sprouts. Trim the stems to a bare minimum (taking care not to slice stem so close that the sprouts fall apart as they cook).

PLACE Brussels sprouts and water in a large microwavable bowl; cover with vented plastic wrap. Microwave on HIGH 7 min. or until sprouts are tender. Plunge sprouts immediately into ice water.

MIX tomatoes and onion in a large microwavable bowl. Microwave, uncovered, 6 min. or until onion is tender and tomatoes are heated through.

CUT Brussels sprouts in half and add to tomato mixture; stir. Cover with plastic wrap. Microwave 3 min. or until Brussels sprouts are hot. Sprinkle with cheese; cover. Let stand 1-2 min. or until cheese is melted.