Remove from oven, spread marshmallows on top and bake 5 minutes more or until marshmallows are lightly browned. Add soup, sour cream, cheese and onion. Top of Page Click above. Remove from oven. Carefully remove the inside of the potato, leaving enough potato to support the skin.
Set potato skins aside.
Place scooped out potatoes into a 3 quart sauce pan. With a potato masher or an electric hand mixer, mash potatoes with a small amount of warm milk, add cheese and butter. Continue mashing adding just enough of the warm milk to make the potatoes soft enough to fill the empty potato skins. Fill each potato skin with the mashed mixture. Place potatoes in a 9x12 baking dish.