Preheat over to 350 Degrees F. Rub onion halves with olive oil. Sprinkle halves on both sides with salt and white pepper. Place onions, cut side down, in a large skillet that can go into the oven. Baken onions until soft, about an hour.

With a spatula, scoop up onions and place them in a serving dish, cut side up. Place water, vinegar, brown sugar and butter in same skillet that onions were roasted in. Srape skillet to dissoslve brown, baked-on juices from onions. Heat to boiling. Boil a minute to reduce, then spoon mixture over each onion. Taste and add more salt if necessary. Serve hot.