1. In a heavy 12-inch skillet, melt the 2 tablespoons of butter with the ¼ cup of olive oil over moderate heat.
2. Add the onions and cook them, turning them frequently, for 10 minutes, or until they are soft and lightly browned. Stir in the peppers, reduce the heat, cover the skillet and cook for 10 minutes.
3. Add the tomatoes, vinegar, salt and a few grindings of black pepper; cover and cook for another 5 minutes.
4. Then cook the vegetables uncovered over high heat, stirring gently, until almost all the liquid has boiled away.
5. Serve the peperonata as a hot vegetable dish, preceding or along with the main course, or refrigerate it and serve it cold as part of an antipasto or as an accompaniment to cold roast meats or fowl.
