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.
Notes
Pick any favorite dressing and toss with the blanched and cooled green beans. If you’re feeling hurried or just wanting to keep things simple, some fresh lemon juice, a generous splash of good olive oil, and a sprinkling of some slightly coarse salt will give you a side dish to be proud of.