There are few vegetables more homely or humble than turnips. If you don't like their assertive flavor, look for baby white turnips, which have a milder, sweeter taste than their more mature counterparts. Otherwise, choose small to medium turnips so that their peppery piquancy does not overwhelm this dish. Apple cider can be difficult to source in some parts of the country, but apple juice makes an acceptable substitute.
Position a rack in the middle of the oven and preheat to 375 degrees F.
Cook the bacon in a medium skillet over medium heat until the bacon is crisp. Drain off the fat and reserve the bacon at room temperature.
Combine the crème fraîche, heavy cream, and apple cider in a large saucepan and bring to a simmer over low heat, stirring occasionally. Add the turnip slices, raise the heat to medium, and cook until the turnips are hot, 5 to 8 minutes. Stir in half of the bacon.
Transfer the mixture to a 7x11-inch baking dish and scatter the remaining bacon atop the gratin. Cook until the turnip is tender and the gratin is golden and bubbling, 35 to 40 minutes. Serve hot.