Preheat oven to 375 degrees then coat a 2-qt baking dish lightly with cooking spray.

Place a medium saucepan over medium heat then add cream, butter, garlic, thyme, salt and pepper. Bring mixture to a boil then remove from heat. Let mixture rest while you prepare turnips. Trim root and stem end from each turnip then peel using a vegetable peeler. Use a sharp knife or a mandolin to slice each turnip into 1/8" thick slices and set aside.

Layer ¼ of the sliced turnips into the bottom of the prepared baking dish by shingling the slices then sprinkle a quarter of the cheese over the layer of turnips. Strain cream mixture to remove the garlic and thyme then pour ¼ of the cream evenly over the cheese. Repeat the process 3 more times with the remaining ingredients.

Cover dish with foil, place on a baking sheet then bake for 40 minutes. Remove foil then bake for an additional 20–25 minutes, until the top is golden and the turnips are tender. Test doneness by inserting a paring knife into the center of the gratin. The turnips will feel slightly firm but not crunchy. Let rest 10 minutes before serving.