Preheat oven to 190°C. Grease a large glass baking dish with Ladismitter Cultured Butter. Whisk cream, onions, rosemary, salt and pepper in a medium bowl to blend.
Place half of the potatoes in a prepared baking dish, overlapping slightly. Sprinkle with ¾ cup Ladismith White Cheddar. Top with second layer of potatoes. Pour cream mixture over potatoes in dish. Sprinkle with remaining Ladismith White Cheddar.
Cover gratin with foil and bake for 1 hour. Uncover and bake until top is golden brown and potatoes are tender, about 45 minutes longer. Let gratin cool 10 minutes before serving.