Place 5 cups of water to boil with salt, once the water has reached a rolling boil, remove from the heat and add the string beans to the water for about 5 minutes. Strain the string beans through a colander and run under cold water.
In a large bowl, whisk the shallots, mustard and vinegar together. Add the string beans and some salt and pepper and toss to combine. Serve chilled or at room temperature, drizzled with a little extra-virgin olive oil, if you like.