Fill a large bowl with ice water. Bring a large pot of water to a boil and add 1 tablespoon of salt. Add the beans and cook for 1 minute only. Time it carefully! You want the beans to be crisp-tender. Drain the beans and put them in the ice water until completely cool. Drain the beans again, dry on paper towels, and place them in a large bowl.

In a small bowl, whisk together the mustard, vinegar, 1/2 teaspoon salt, and the pepper. While whisking, slowly add the olive oil to make an emulsion.

Pour enough dressing over the beans to moisten them well, reserving the rest for another use. Toss with the dill, season to taste, and serve at room temperature. (If the beans aren't served immediately, refrigerate them and add a little extra vinaigrette and a sprinkle of salt before serving.)