1. Prepare grill.
2. Coat squash mix and onion evenly with cooking spray. Place vegetables on grill rack; grill 4 minutes on each side or until crisp-tender and beginning to brown.
3. Place tomato in a medium bowl; add dressing and basil, tossing gently to coat. Add cooked vegetables to tomato mixture; toss well.
4. Coat cut sides of bread with cooking spray. Grill bread 1 minute on each side or until lightly toasted. Spoon vegetable mixture over bottom half of bread; sprinkle evenly with cheese. Top with remaining bread half. Press down lightly; cut crosswise into 4 equal pieces.