2 medium-large Idaho potatoes
1 pound string beans, trimmed
3 tablespoons olive oil
3 tablespoons white wine vinegar
¼ teaspoon salt
freshly ground pepper, to taste
1 small red onion, thinly sliced
Boil the potatoes in 2 quarts of salted water for 20 minutes. Add the beans and boil 7 minutes longer, until the beans and potatoes are just tender. Strain into a colander, remove and set aside the potatoes. Refresh the beans under cold running water and drain well. When the potatoes are cool enough to handle, peel and slice them 1/3-inch thick. If the beans are long, halve them crosswise.
In a large bowl, whisk together the oil, vinegar, salt, and pepper. Add the onion, potatoes, and beans, and toss very gently to dress all ingredients without breaking the potatoes.