PREHEAT OVEN TO 190ºC . TOSS together turnips, potatoes, onion with melted butter and place in a 23 cm square or round baking dish. Cover tightly and place in preheated oven for 30 minutes. In a small pot on top of the stove combine milk, nutmeg, pepper and salt and bring to a boil. Immediately remove from heat. Remove turnip-potato mixture from the oven, remove cover and mix in half the cheese. Pour the milk over the potatoes and sprinkle with remaining cheese. Replace in oven, uncovered, another 20 to 25 minutes. If the gratin is golden brown, it's ready to serve. If not, preheat broiler. Place gratin under broiler about 3 minutes to brown top before serving.