Position a rack in the center of the oven and heat the oven to 450°F. Coat a large rimmed baking sheet with 1 Tbs. of the oil.
In a large bowl, toss the potatoes, 2 Tbs. of the oil, 1 tsp. salt, and 1/2 tsp. pepper. Arrange in a single layer on the prepared baking sheet. Roast until barely tender, about 20 minutes, gently tossing with a spatula halfway through.
Meanwhile, toss the mushrooms, 1 Tbs. of the oil, 1/2 tsp. salt, and 1/4 tsp. pepper in the bowl.
Gently toss the potatoes again, scatter the mushrooms evenly over the potatoes, and continue to roast until both the potatoes and mushrooms are tender and golden brown in spots, 10 to 15 minutes more.
Meanwhile, in a medium bowl, whisk the mascarpone, orange juice, vinegar, 1 tsp. salt, and 1/8 tsp. pepper. Drizzle in the remaining 2 Tbs. oil while whisking constantly; set aside.
Return the potatoes and mushrooms to the large bowl and add the mascarpone dressing. Toss gently to coat, season to taste with salt and pepper, and transfer to a large serving bowl. Garnish with the scallions and serve.