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