Step 1 of 5

Heat the oven and prep the dish

Heat the oven to 375°F. Rub a baking dish all over with the butter. Peel the potatoes and slice them thin — about ⅛ inch (3 mm). A mandoline makes quick, even work of it, but a steady hand and a sharp knife are fine.

— Even slices matter more than thin ones here — a mix of thick and thin bakes unevenly, leaving some slices raw and others collapsed.