How to Make It
Preheat the oven to 400°. Position a rack in the upper third. On a large rimmed baking sheet, toss the potatoes with 1 tablespoon of the olive oil and season with salt and pepper. Spread the potatoes in a single layer and roast for 12 minutes, turning once, until barely cooked. Add the sausages and roast for about 10 minutes longer, turning them once, until the potatoes are tender and the sausages are lightly browned and heated through. Blot the potatoes and sausages dry with paper towels and cut the sausages into 1/3-inch-thick slices. Transfer to a large bowl.
Meanwhile, in a small bowl, whisk the vinegar, shallot and mustard. Slowly whisk in the remaining 1/4 cup of olive oil and season with salt and pepper. In a medium bowl, toss the arugula with 1 tablespoon of the dressing and a pinch each of salt and pepper. Transfer to a platter. Add the rest of the dressing to the potatoes and sausages, then toss in the scallion. Add the blue cheese and toss quickly. Mound the potato salad over the arugula and serve.