The perfect addition to a charcuterie board. The onions are not too sweet or vinegary.
In a large sauté pan, heat the olive oil over medium-high heat for a few minutes. Add the onions and season with salt. Reduce the heat to medium-low, cook, and stir occasionally until the onions are very soft but not taking on any color, about 10-15 minutes.
Add the vinegar and continue to cook, stirring, until the onions break down to a jammy consistency, about another 20 minutes. Taste for seasoning. Let cool. Onions can be kept in the refrigerator for up to 2 weeks in an airtight container.