Arrange the onion wedges on a quarter-sheet pan. Drizzle with olive oil, sprinkle with salt, and crack a good amount of pepper over the onions. Roast for 30-35 minutes, or until soft and beginning to brown in places. Drizzle with balsamic vinegar.
Notes
True, syrupy balsamic vinegar can be very pricey; as an alternative, make balsamic reduction out of a less expensive bottle.
Recipe by A Modest Feast at http://www.amodestfeast.com/2016/04/13/roasted-red-onions-thyme/