Roast the bell pepper. Pull off the loosened skin and remove the core, seeds and ribs. Cut the pepper into long, thin strips. Set aside.

Fill a saucepan three-fourths full with water and bring to a boil. Add the onions and 1 tbsp of the salt and bring back to a boil. Reduce the heat slightly, cover partially and cook for 2 - 3 minutes; drain and plunge into cold water. Cut off the root ends; trim the stems if you like. Slip off skins and set the onions aside.

Trim off the stems from the green beans and cut the beans into 3-inch (7.5-cm) lengths. Bring a large saucepan of water to a boil. Add the remaining 2 tablespoons salt and the beans. Bring back to a rapid boil and cook, uncovered, until just tender but still crisp, about 5 minutes. Drain immediately and plunge into ice water to stop the cooking and to cool. Drain again and set aside.

Just before serving, combine 1 tbsp of the vinegar and the pinch of salt in a small bowl and stir to dissolve the salt. Add the olive oil, then pepper to taste and whisk together. Pour into a saucepan and warm over medium heat. Add the bell pepper, onions and beans and toss until heated through, 1 minute. Taste for seasoning, adding more vinegar or salt if needed.