Instructions

• Heat 2 tablespoons olive oil in a large skillet over medium heat until the oil shimmers.
• Add 4 large onions (thinly sliced, about 10 cups) to the skillet and spread them into an even layer.
• Cook the onions, stirring every 3–5 minutes, until they are soft, translucent, and just beginning to brown, about 20 minutes. If they brown too quickly, reduce the heat to medium-low.
• Stir in 3 tablespoons balsamic vinegar, 2 tablespoons brown sugar, 1 1/2 teaspoons finely chopped fresh rosemary (or 1/2 teaspoon dried), 1/4 teaspoon kosher salt, and 1/4 teaspoon freshly ground black pepper, scraping up any browned bits from the pan.
• Continue to cook, stirring every few minutes, until most of the liquid has evaporated, the onions are dark brown, and the mixture is reduced to about 3/4 cup, about 40 minutes more. Adjust the heat as needed to keep the onions from burning.
• Remove the skillet from the heat and let the onion jam cool to room temperature.
• Transfer the cooled jam to a glass jar or airtight container and refrigerate for up to 1 week.