Put butter, brown sugar and sliced onions into a large pot over medium heat. Cook until onions are dark brown and almost mush, about 45 minutes to an hour, stirring from time to time.
Slice your tators to about 1/8 to 1/4 inch thick. Put one layer of potato rounds in the bottom of a rectangular casserole dish. Put a thin layer of caramelized onions on top followed by a sprinkle of rosemary, a sprinkle of cheese, a drizzle of cream and a pinch of salt and pepper. Repeat the layers until you are out of potatoes or onions. I usually get about three layers but it would depend on the size of your casserole dish.
Cook at 375 for 50 minutes to an hour. Test tator layers with a fork to make sure they are done. Remove from oven and let cool for 5 minutes.