This is great for holiday dinner tables. Use a mandolin or a very thin food processor blade to get thin uniform potato slices. When the slices are the same size the gratin cooks more evenly. You can also make this recipe a day ahead and reheat it.
Preheat oven to 350 degrees. Brush the sides of a 9x13 inch dish with the melted butter. Pour the cream into a large bowl. Slice the potatoes into 1/4 inch slices preferably with a mandolin or food processor and place them in the cream as you slice them.
Remove half of the potatoes from the cream and layer them in a prepared pan. Sprinkle with the garlic, half of the gruyere cheese and all of the rosemary. Season the top lightly with season salt and pepper. Layer with the rest of potatoes and top with the remaining cheese.
Place the dish on a baking sheet and place it in the oven. Bake until the top is golden brown and the potatoes are tender about one hour.