Add potatoes to a medium pan and cover with water. Bring to a boil, then cook for 15-20 minutes. Drain and allow potatoes to cool. Set aside.

Fill a large pot with water and bring to a boil. Add green beans and cook until bright green, about 5 minutes. Drain green beans in a colander and rinse with cold water until cool.

In a large mixing bowl, combine potatoes, green beans, bell pepper and onion.

Add mustard, vinegar and salt. Mix until vegetables are evenly coated. Allow flavors to develop for at least 20 minutes before serving.