Cut the potatoes in half lengthwise and place them on a sheet pan. Add 2 tablespoons olive oil, 1 teaspoon salt, 1/4 teaspoon pepper and the mustard to the potatoes and toss well with your hands.
Roast until the potatoes are tender and browned all over, tossing occasionally, 30 to 40 minutes. Transfer to a medium bowl.
Whisk together the dill, parsley, vinegar, remaining 1 tablespoon olive oil, 1/2 teaspoon salt and 1/4 teaspoon pepper in a small bowl. Toss with the hot potatoes and serve hot, warm or at room temperature.