Mashed Potato Gratin

This holiday side combines two classics: mashed potatoes and cheese-topped potato gratin. Milk-simmered mashed potatoes are the base, sliced potatoes and shredded Gruyère are layered on top, then the casserole is baked until golden. If you own a mandoline, here’s a good time to use it, but hand-slicing with a knife works, too. Aim for ⅛-inch thickness so you have enough for full coverage and so the spuds cook through in the time indicated. If you need a short pause in the preparation to focus on other dishes, cover the mashed potatoes with foil after spreading them in the baking dish; they’ll hold at room temperature for up to an hour before the casserole needs to be finished.