In a large bowl, whisk the mustard with the vinegar. Gradually whisk in the olive oil and season with salt and pepper. Add the green beans, tarragon, chives and thyme and toss to coat. Add the tomatoes, toss gently and serve.
The salad can be refrigerated for up to 4 hours.
A Niçoise salad isn't complete without the traditional accompaniments: mixed olives, anchovies, hard-boiled eggs and steamed potatoes tossed with olive oil and fresh herbs.
The French Riviera is famous for fragrant, light-bodied, dry rosés made from Carignane, Cinsaut, Grenache and Mourvèdre grapes. Look for one with crisp acidity and fresh berry flavors to serve with this dish.