Wash and clean potatoes. Boil in generously salted water until tender. rain; cool to room temperature.
Wash and trim green beans; break into two or three pieces. Steam or boil green beans in slightly salted water until slightly tender but not too soft. rain; cool to room temperature.
In a large bowl, add cooled green beans, potatoes, and onion; add oil, vinegar, salt, and pepper and toss until vegetables are coated with dressing. Serve chilled.
A great summer dish that can be made with small yellow or red potatoes and red or white wine vinegar. Add a pinch or two of finely minced garlic for an extra burst of flavor.