Bring a large pot of salted water to a boil. While the water is coming to a boil, fill a large bowl with ice water. In a small bowl combine the vinegar, tarragon and shallots. Set aside.
Plunge the beans into the boiling water and cook for about 4 minutes until just barely tender. Drain and drop them immediately into the bowl of ice water to stop the cooking. Let them cool for a few minutes, then drain well.
Meanwhile, add the olive oil, mustard and salt and pepper to the shallot mixture and stir to combine. Turn the drained and cooled beans into a serving dish or bowl, drizzle over the dressing, and toss to coat the beans. Season with more salt and pepper if needed. Serve at room temperature.