Cut the onions in half lengthwise, leaving the skins on. In a 14-inch skillet, heat olive oil until smoking. Place 4 onion halves cut side down into skillet and cook until dark golden brown, about 7 to 8 minutes. Add vinegar and place in oven and bake until soft, about 15 to 20 minutes. Remove from oven and remove onions from pan immediately placing cut side up on cool plate. Pour syrup that has formed in pan over onions and let cool to room temperature.
