I ate a lot of potatoes as a child, and my Grandmother Rosa had many ways of dressing them up. This was one of my favorites as a kid when my grandmother's earliest string beans were harvested. Try it as picnic fare.
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.2.
Strain into a colander, remove and set aside the potatoes. Refresh the beans under cold running water and drain well.3.
When the potatoes are cool enough to handle, peel and slice them 1/3-inch thick. If the beans are long, halve them crosswise.4.
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.