Preheat oven to 400 degrees F (200 degrees C). Grease a 9x13 inch baking dish. Bring a large pot of water to a boil. Add potatoes and boil until slightly underdone, about 15 minutes. Drain, cool and peel.
In a large saucepan over medium heat, combine margarine, brown sugar, 2 cups marshmallows, cinnamon and nutmeg. Cook, stirring occasionally, until marshmallows are melted.
Stir potatoes into marshmallow sauce. While stirring mash about half of the potatoes, and break the others into bite-sized chunks. Transfer to prepared dish.
Bake in preheated oven for 15 minutes. Remove from oven and cover top evenly with remaining marshmallows. Return to oven and bake until marshmallows are golden brown.
