Preheat the oven at 400 degrees. Pour the cream into a large bowl and add the potato slices. In a sauté pan heat the oil under medium heat and sauté the onion and fennel until soft, about 8 minutes, adding the garlic for the last minute. Add the onion mixture to the bowl along with the salt, pepper, cheese and rosemary. Grease a large casserole dish with butter and pour in the potato mixture from the bowl, spreading evenly in the dish. Bake in the oven for at least 1 hour until bubbling and golden brown on top.