Divide the potatoes, carrots and onion equally among four pieces of heavy-duty aluminum foil (about 18 in. x 12 in.). Top with butter; sprinkle with salt if desired and pepper. Bring opposite short ends of foil together over vegetables and fold down several times. Fold unsealed ends toward vegetables and crimp tightly.
Grill, covered, over medium heat for 10-15 minutes on each side or until potatoes are tender.
Remove from grill. Open foil and sprinkle with cheese; reseal
Let stand for 5 minutes or until the cheese melts.