These roasted browned onions get an added kick from the balsamic vinegar.
Preheat oven to 450 degrees. Line a rimmed baking sheet with aluminum foil. Place onions on sheet; toss with oil, 2 tablespoons vinegar, and rosemary. Season with salt and pepper.
With another piece of foil, cover baking sheet, and seal tightly. Roast until onions have begun to soften, about 30 minutes. Uncover, and continue to roast, tossing occasionally, until onions have softened and browned, about 30 minutes. Toss with remaining vinegar, and serve.