Blanch the green beans: bring a large pot of water to a boil. Add the green beans and cook until they are just tender about 3 to 4 minutes. Use a slotted spoon to remove the beans and transfer them to an ice bath.
In the same pot add the potatoes, and simmer until they are firm but easily pierced with a fork, about 30 minutes. Remove and let cool slightly.
Meanwhile, prepare the vinaigrette: in a small bowl whisk together the vinegar, Dijon, garlic, salt, olive oil, and pepper.
In a large bowl toss the potatoes, green beans and vinaigrette together until well coated. Top with parsley and serve.