Preheat the oven to 350°F In a large saute pan, melt the butter over medium heat until just foaming. Reduce heat to low and saute leeks until very soft, about 10 minutes.
In a large gratin dish, alternate layers of potatoes, leeks, and cheese, seasoning each layer with salt and pepper.
Finish with a final sprinkling of cheese. Pour the cream over top and cover lightly with aluminum foil. Bake for 40 minutes. Uncover and bake for another 30 to 40 minutes, or until bubbling and brown on top. Let cool slightly.