Preheat oven to 450 degrees. If using fresh onions, soak them in warm water for 5 minutes to loosen skins; using a paring knife, cut off ends, and peel onions.
Toss onions with vinegar and oil on a foil-lined rimmed baking sheet; season with salt and pepper. Roast until onions are tender, 25 to 30 minutes, stirring 2 or 3 times and rotating baking sheet halfway through.
True balsamic vinegar is made by aging boiled Trebbiano grape juice in barrels for 12 to 25 years and can cost $50 to more than $200. Many versions at supermarkets are made from regular wine vinegar with added sweeteners -- they cost $3 to $14.