- 2 tablespoons olive oil
- 4 large onions (about 10 cups), thinly sliced
- 3 tablespoons balsamic vinegar
- 2 tablespoons brown sugar
- 1 1/2 teaspoons fresh rosemary , finely chopped
- 1/4 teaspoon kosher salt
- 1/4 teaspoon freshly ground black pepper
- Preheat oil in a large skillet over medium heat. Add onions. Cook, stirring occasionally, until onions are soft, translucent, and just beginning to brown, about 20 minutes.
- Stir in balsamic vinegar, brown sugar, rosemary, and salt and pepper. Continue to cook, stirring occasionally, until liquid is evaporated and onions are dark brown and reduced to 3/4 cup, about 40 minutes more.
- Remove from heat and let cool. Store in a glass jar or airtight container in the refrigerator for up to 1 week.