1. Preheat oven to 375°F (190°C).
2. Coat unpeeled onions with olive oil.
3. Place onions in a medium-sized cast-iron skillet.
4. Bake in oven 1 hour or until soft. Remove onions from skillet. Cut in half lengthwise; arrange cut side up on a serving platter.
5. Deglaze skillet with vinegar, scraping up the juices. Reduce until syrupy, about 5 minutes.
6. Pour over onions and serve at room temperature.