1) Preheat the oven to 210°C.
2) Blanch the green beans in a large pot of boiling salted water for just 4 minutes. Drain immediately and immerse in a large bowl of ice water to stop the cooking. When they are cool, drain and set aside.
3) Meanwhile, in a large bowl toss the onion and peppers together with 2 tablespoons of olive oil and sprinkle generously with salt and pepper.
4) Place the onion and peppers in a single layer on a baking tray and roast for about 15 minutes, tossing with a spatula from time to time to be sure the vegetables roast evenly.
5) Just before serving, reheat the string beans in a large saute pan drizzled with a little olive oil. Sprinkle with salt and pepper and arrange on a platter.
6) Spoon the roasted vegetables over the string beans and serve hot or at room temperature.