In a large bowl, toss the potatoes with the oil and salt. Spread in a single layer on a parchment-lined baking sheet and roast in a preheated 425 degree oven for 40-45 minutes or until browned, crisp and tender.
Meanwhile, to prepare dressing, in a large bowl, combine vinegar, garlic, salt and mustard. Whisk in oil. Stir in parsley.
Add hot potatoes to dressing and toss. Serve warm or at room temperature on a bed of arugula.