PREHEAT grill to medium-high heat. Place onions in center of (24x18-inch) sheet of foil; top with squash and tomatoes. Sprinkle with basil.
BRING up foil sides. Double fold top and ends to seal, leaving room for heat circulation inside.
GRILL 15 min. or until vegetables are tender. Open top of packet; sprinkle vegetables with cheese. Let stand 3 min. or until cheese is melted.