1.) Place the potatoes and enough water to cover them in a large saucepan.
2.) Bring to boil.
3.) Reduce the heat and simmer, covered, until tender, 15-20 minutes; drain.
4.) When potatoes are cool enough to handle, cut them into 1/2-inch slices.
While potatoes are cooking:
5.) Whisk together the vinegar , mustard, oil and salt in a large bowl.
After potatoes are cooled and sliced:
6.) Add warm potatoes to sauce and gently toss to coat
7.) Add the arugula, shallots, and scallions. Toss gently and either serve warm or refrigerate and serve later