The verdant taste of crisp-tender haricots verts (slender French green beans) combined with tangy Dijon mustard makes for a perfect picnic side dish. If you can't find haricots verts, use the thinnest green beans available.
In a small bowl, whisk together mustards and the vinegar. Whisking constantly, add the oil in a steady stream; continue whisking until mixture is smooth and emulsified.
Prepare an ice bath; set aside. Bring a large pot of water to a boil; add salt. Add beans, and cook until crisp-tender, about 5 minutes; drain. Transfer beans to an ice bath to stop cooking; drain well.
Toss beans with onion and vinaigrette. Season with salt and pepper. Serve at room temperature.