- 2 tablespoons unsalted butter
- 2 tablespoons extra-virgin olive oil
- 2 large red onions, halved and thinly sliced
- 1 cup water
- 1/2 cup balsamic vinegar
- 3 tablespoons sugar
- 1 rosemary sprig
- Kosher salt
- In a medium saucepan, melt the butter in the olive oil. Add the onions and cook over moderate heat until softened, about 7 minutes. Add the water, balsamic vinegar, sugar and rosemary and season with salt. Cover partially and simmer over low heat until tender, about 30 minutes. Uncover the onions and simmer until the juices are syrupy, about 15 minutes longer. Discard the rosemary and serve warm.
The onions can be refrigerated overnight. Reheat gently.