Fill a large pot of salted water and bring to a boil. Prepare an ice water bath in a separate large bowl and set aside.

Cook the haricots verts in the boiling water for 4 – 5 minutes, just until bright green and tender-crisp.

Drain the beans and immediately transfer them to the ice water bath to stop the cooking process. Drain well.

To serve the beans warm, heat 1 additional tablespoon of olive oil in a large pan over medium heat and add the drained beans, sauteing for 2-3 minutes until warmed through. This step isn't necessary if you plan to serve the beans cold though.

In a small bowl, whisk together the shallot, olive oil, vinegar, and mustard. Taste and season with salt and pepper as needed.

Drizzle the vinaigrette over the beans in a large bowl and toss to coat evenly. Place on a platter and serve.