This gratin is a rich, warming treat for the coldest winter evenings, with sweet parsnips enrobed in cream and tucked under a generous blanket of molten cheese.
Preheat the oven to 350°F. Butter a 13 × 9-inch baking dish. Peel and trim:
If the parsnips are thin, halve them lengthwise. If they are larger, cut them lengthwise so they are in roughly ½-inch-thick batons. If the parsnips have a woody core, cut it out. Set aside. Have ready:
Mix together in a medium bowl:
Scatter the onion over the bottom of the dish and place the parsnips on top. If using, top with the prosciutto, then sprinkle with the cheese. Pour the cream mixture over everything. Bake until the top is golden and the parsnips are tender, 45 to 55 minutes. If desired, sprinkle halfway through baking with: