Bring a large pot of salted water to boil. Add the green beans and cook until just tender, 2 to 3 minutes. Transfer to a bowl of ice water using tongs, pushing the beans down to submerge them. Let cool completely, about 5 minutes, then drain and pat dry.
Meanwhile, whisk the vinegar, sugar and 1 teaspoon salt in a large bowl. Add the shallots and stir to coat. Let sit until the green beans are ready.
Whisk the mustard into the shallot mixture, then gradually add the olive oil, whisking constantly, until smooth and thick. Add the green beans and toss to coat. Season with salt and pepper.