Prepare grill for indirect heat. Cut 1/4 in. from top and bottom of each onion; peel. Core onions, leaving the bottom fourth of the onions intact. Place each onion on a double thickness of heavy-duty foil (about 12 in. square). Place bouillon in each onion; top with butter and seal tightly., Grill, covered, over medium heat for 35-40 minutes or until onions are tender., Carefully unwrap; top each onion with a cheese slice. If desired, top with fresh thyme. Serve immediately.