Cook potatoes in salted water until just tender when pierced with a knife. Drain. While still hot, peel and cut into large pieces. Mix with oil and vinegar, then season with salt and pepper to taste. Set aside to cool.

Cook green beans until barely tender (if using fresh beans). Drain and immediately place in cold water to cool them quickly. Drain again. Gently mix beans (cooked or thawed frozen green beans) and diced onion into potatoes. Garnish if desired with anchovies, egg and capers.