Put the potatoes in a saucepan with enough lightly salted water to cover by 2 inches. Bring to a boil over medium-high heat. Reduce the heat to medium-low and boil gently until tender when pierced with the tip of a knife, 20 to 30 minutes. Meanwhile, drain, but do not rinse, the beans and place the onion slices in a bowl with ice water to cover.
In a salad bowl, whisk together the vineg