Place potatoes in a saucepan, along with water enough to cover the potatoes by 1". Bring to a boil, lower the heat and simmer the potatoes until fork tender, about 12~15 minutes.
In the meantime, whisk the mustard, mayonnaise, salt and pepper in a mixing bowl.
Drain the potatoes well and add them to the bowl along with scallions and parsley. Mix well and serve at room temperature.