with a sharp knife, carefully remove a 1 inch core from the center of each onion..
Place each onion on a double thickness of heavy duty foil, about 12 inches 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 to 40 minutes or until they are tender. Open foil carefully to allow steam to escape. Serves four.