When you use low-fat milk for a gratin, you will find quite a bit of liquid in the pan when you pull the dish from the oven. Let it sit for 20 to 30 minutes and the turnips will reabsorb the moisture. If any liquid remains in the dish, it’s delicious spooned over the gratin.
Preheat the oven to 400 degrees. Butter or oil a 2-quart baking dish or gratin dish. Rub the sides and bottom with the cut clove of garlic.
Place the sliced turnips in a bowl and season generously with salt and pepper. Add half the cheese and the thyme and toss together, then transfer to the gratin dish and pour on the milk. It should just cover the turnips.
Place in the oven and bake 30 minutes. Push the turnips down into the milk with the back of a large spoon. Sprinkle the remaining cheese over the top and return to the oven. Bake another 40 to 50 minutes, until all of the milk is absorbed, the turnips are soft and the dish is nicely browned on top and around the edges.