Scrub potatoes with a stiff brush under running water. Cut potatoes in quarters. In a Dutch oven cook potatoes in a small amount of boiling, lightly salted water, covered, for 10 to 15 minutes or until tender. Add beans the last 3 to 5 minutes of cooking. Drain; rinse with cold water until vegetables are cooled. Drain.
In a medium bowl stir together oil, vinegar, rosemary, salt, and pepper. Add potato mixture and onions. Toss mixture gently. Serve immediately. Makes 6 to 8 side-dish servings.