Bring a large pot of salted water to a boil. Prepare a large bowl of ice water and place nearby. Add the haricots verts to the boiling water and cook for 4 – 5 minutes or until bright green and tender-crisp.
Drain the beans and quickly plunge them into the ice water. Let sit for 2 minutes or until cool; drain. Pat dry and place in a large bowl.
In a small bowl, whisk together the shallot, mustard, and vinegar. Whisk in 3 tablespoons of the olive oil. Taste – if it’s too spicy you can add the additional tablespoon of olive oil. Season with salt and pepper.
Drizzle the vinaigrette over the beans and toss to coat evenly; season with salt and pepper. Place on a platter and serve immediately.
SERVE WARM: Boil the beans for less time initially – about 2 minutes – then chill. Just before serving, heat a tablespoon of olive oil or butter in a large saute pan and add the beans. Stir or toss for 2-3 minutes or until warmed through. Remove from heat and toss with the vinaigrette. Season with salt and pepper and serve immediately.
MAKE AHEAD: Blanch the beans a day ahead; wrap in paper towels and store in Ziploc bag in the refrigerator. Make the vinaigrette and toss with the beans just before serving.