2. Spread the onion slices (try to keep them intact) in a single layer on a baking sheet. Brush them with olive oil and sprinkle with salt.
3. Roast the onions on the top rack of the oven until the onions are completely soft, about 10 minutes. Sprinkle the vinegar over the onions and return to the oven. Continue to roast until the vinegar coats the onions like a glaze, about 5 minutes.
4. Serve hot with other roasted vegetables or meats. Store in the refrigerator for up to 5 days and use as a garnish on salads or sandwiches.