Preheat the oven to 325 F. Generously grease a gratin dish with butter. Layer the the potatoes and mushrooms in the dish, sprinkling each layer with the onion, parsley, cheese, garlic, and seasoning. Combine the cream and water in a small sauce pan and bring to just under the boiling point. Pour over the dish. Sprinkle the butter and cheese over the top. Bake for 1 1/2 hours or until the potatoes are done.{$7e}(Original recipe for 6)