Thinly slice potatoes on mandoline into a large bowl and toss well with melted butter mixture.

Transfer potatoes to baking dish and spread out evenly. Cover tightly with foil and bake in middle of oven until potatoes are tender when pierced with a knife, 30 to 40 minutes.

Remove foil and bake, uncovered, until browned, 35 to 45 minutes.

Make ahead: Potatoes can be baked 4 hours ahead and kept at room temperature. Reheat, uncovered, in 350 F. oven, about 15 minutes. Potatoes can be reheated while roast is standing and being sliced.