1 1/2 pounds red potatoes
1/4 pound green string beans, stems removed
2 tablespoons finely chopped shallots
1 1/2 tablespoons whole-grain or Dijon mustard
1 tablespoon red wine vinegar
1/2 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
3 tablespoons olive oil
2 tablespoons chopped scallions, optional
Place the potatoes in a medium pot and cover with water. Bring to a boil over medium-high heat, and cook until a knife can be easily inserted into the center of the potatoes, 15-20 minutes. Drain and set aside to cool. When cool enough to handle, cut into bite-sized cubes.
Bring a small pot of water to a boil and add the string beans. Cook until crisp tender, 1-2 minutes. Drain in a colander and cool the beans under running cool water. Cut into 2-inch pieces.
In a medium bowl, whisk the shallots, mustard, vinegar, salt, pepper and oil together. Add the potatoes, beans and scallions, if using, and gently toss together.
Tip: To keep the string beans crisp, rinse them in cold water or soak them in ice water right after cooking. This will refresh them and stop the cooking process.