Pour the remaining vinaigrette into a glass jar with a lid and store in the refrigerator until ready to use.
To cook the white beans: Put the beans into a pot and add enough water to cover the beans by 4 inches. Either soak overnight, or for a quick soak, set the pot over high heat and bring to a boil. Turn off the heat and allow the beans to soak for at least 1 hour.
Once the beans have soaked, drain them. Return beans to the pot and add enough water to cover the beans by 4 inches. Set over high heat, bring to a boil, reduce the heat, cover, and simmer the beans until tender, about 45 minutes to 1 hour (depending on the age of the beans — older beans take longer to cook). Allow to cool in the cooking liquid, then drain the beans and set aside until ready to use.
To grill the steak and compose the salad: Heat the grill to medium high or heat the broiler. Grill or broil the flank steak with the peppers about 2 to 4 minutes per side, basting both the peppers and the steak with a little of the marinade.
Transfer the steak to a plate and let it rest for about 5 minutes. Thinly slice the meat against the grain, saving the juices.
Arrange the sliced steak with the juices, cooked beans and the charred red peppers on a serving platter. Drizzle with the reserved vinaigrette. Serve at room temperature.