Preheat the oven to 400 degrees.

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 for 30-40 minutes, tossing occasionally, until the potatoes are tender and browned all over. Transfer to a medium bowl.

In a small bowl, whisk together the dill, parsley, vinegar, 1 tablespoon olive oil, and ½ teaspoon salt. Toss with the hot potatoes, and serve hot, warm, or at room temperature.