Use turnips, rutabagas, kohlrabi or a mix of these vegetables in this delicate winter gratin. 1 1/2 pounds turnips, kohlrabi, rutabaga, large parsnips or a combination, peeled and sliced thin (if the rutabagas are very large, cut them in half and slice in half-moons; slice the turnips and kohlrabi into rounds; quarter and core the parsnips, then slice them) Salt and freshly ground pepper 1/2 teaspoon fresh thyme leaves 3/4 cup grated Gruyère cheese (3 ounces) 1 1/2 cups low-fat (1 percent or 2 percent) milk 1. Preheat the oven to 400 degrees. Butter a 2-quart gratin or baking dish. Place the sliced vegetables in a large bowl, and season generously with salt and pepper. Add the thyme, and toss together. 2. Arrange the vegetables in the gratin dish. Add the milk, season with more salt and pepper if you wish, and place in the oven on the middle rack. Bake 45 minutes; every 15 minutes, press the vegetables down into the milk with the back of your spoon. Add the cheese, and stir in carefully to incorporate. Return to the oven, and bake another 30 to 45 minutes, stirring or pressing the vegetables down with the back of your spoon every 10 minutes until the gratin is nicely browned and most of the liquid is absorbed. Remove from the heat and serve, or allow to settle and serve warm. Yield: Serves four. Advance preparation: You can make this several hours ahead of serving, and reheat in a medium oven. Nutritional information per serving: 200 calories; 8 grams fat; 5 grams saturated fat; 28 milligrams cholesterol; 22 grams carbohydrates; 6 grams dietary fiber; 167 milligrams sodium (does not include salt to taste); 12 grams protein