Preheat the oven to 425°F.
Add the white balsamic vinegar and sugar to a small saucepan. Heat over medium low until reduced. If you scrape along the bottom of the pan, the line should remain.
Chop the potatoes and onions into 1-inch pieces.
Combine the potatoes and onions with the vinegar reduction, oil, thyme, and salt in a mixing bowl.
Spread evenly on a baking sheet. Roast for 25 minutes, stirring halfway through.