With a sharp knife, carefully remove a 1-in. core from the center of each onion. Cut each onion into four wedges to within 1/2 in. of root end.
Place each onion on a double thickness of heavy-duty foil (about 12 in. square). Place bouillon in the centers of onions; top with butter, thyme, salt and pepper. Drizzle with wine if desired. Fold foil around onions and seal tightly.
Prepare grill for indirect heat. Grill onions, covered, over indirect medium heat for 35-40 minutes or until tender. Open foil carefully to allow steam to escape.