Step 1Cut the onions in half crosswise. Place them in a bowl and add cold water to cover. Let stand for about 1 hour. Drain and pat dry.
Step 2Heat the oven to 350 degrees. Line a baking sheet with heavy-duty aluminum foil and coat with oil. Brush the onion halves all over with olive oil and place them, cut-side up, on the prepared pan. Sprinkle with salt and roast until thoroughly tender and nicely browned, about 1 hour.
Step 3Transfer to a serving dish. Sprinkle lightly with vinegar and generously with pepper to serve.