How to Make It
Preheat the oven to 425°. Cut the potatoes into 1/2-inch wedges. Scatter the potato wedges on 1 large rimmed baking sheet, drizzle with 1 1/2 tablespoons of the olive oil and toss until coated. Season with salt and pepper and roast for about 25 minutes, until browned and crisp.
In a small bowl, whisk the remaining 2 tablespoons of olive oil with the mustard and vinegar and season with salt and pepper. In a large bowl, toss the potatoes with the onion and arugula. Top with the dressing, toss again and serve right away.