How to Make It
In a large bowl, whisk the olive oil with the lemon juice, mustard, shallot and tarragon. In a medium saucepan of salted boiling water, blanch the beans for 2 minutes. Using a slotted spoon, transfer the beans to a bowl of ice water to cool; drain and pat dry. Halve the beans and add to the vinaigrette.
Add the potatoes to the boiling water and cook until tender, about 10 minutes. Drain, then add to the vinaigrette. Season with salt and pepper, toss and serve.