1. Drop the haricots verts into a large pot of boiling salted water. Cook just until they are tender but crisp, about 2 minutes. Immediately plunge the beans into ice water to stop the cooking process. Set aside.
2. In a small bowl, combine the vinegar with 1 tablespoon of the parsley and the shallot and mustard. Gradually whisk in the olive oil. Season with salt and pepper to taste.
3. Drain the beans and place them in a large bowl. Add about half of the dressing and toss to coat the beans thoroughly. Arrange the beans on a serving plate and sprinkle with the onion and the remaining parsley. Serve the remaining dressing on the side.