Heat the olive oil and butter in a large skillet over medium-low heat.
Add the sliced onions and salt. Saute until the onions are very soft and caramelized
Stir in the brown sugar and cook for 1 minute.
Add the apple cider vinegar and continue cooking for 8–10 minutes, stirring frequently, until the vinegar has reduced and the onions are thick, glossy, and jam-like.
Season with black pepper and allow the onion jam to cool completely.