Pile this jam over thinly sliced steak on a soft onion roll for a delicious sandwich.
In a large saucepan with a lid, heat oil over medium. Add onions, season with salt and pepper, and toss to coat. Cover and cook, stirring occasionally, until onions are very soft, 20 to 25 minutes.
Add vinegar and sugar; simmer over medium-low heat, uncovered, stirring occasionally, until syrupy and jam-like, about 30 minutes. Let cool.