This is your chance to try oyster or cremini mushrooms, if you haven't already, in this warm salad.
Preheat oven to 450 degrees, with racks in upper and lower thirds. On a rimmed baking sheet, toss mushrooms with 1 tablespoon oil; season with salt and pepper. On another rimmed baking sheet, toss potatoes with 1 tablespoon oil; season with salt and pepper. Roast until mushrooms are browned and potatoes are cooked through, about 20 minutes, tossing once and rotating sheets halfway through. Transfer to a bowl and toss with parsley, vinegar, and capers, if desired.