Preheat oven to 375 degrees F.
Spread the mashed potatoes into a buttered 1 quart gratin dish. If a flat effect is desired, cover the potatoes with plastic wrap and press evenly flat with your hands. Then remove plastic wrap, season with salt and pepper, to taste, and sprinkle with Parmesan.
Bake until top is golden and crispy, about 20 to 30 minutes. Serve with reheated leftover gravy