Place the chopped & peeled sweet potatoes into a large saucepan, and cover with water by 1 inch.Bring to a boil, then reduce to a simmer, and cook until the sweet potatoes are tender, about 15 to 20 minutes.Drain, and return them to the pan. Because you don't want any extra moisture from the potatoes in your casserole, heat the potatoes over medium heat, stirring, until liquid has evaporated and a thin film covers bottom of pan, about 2 minutes.Remove the pan from the heat and add the milk, butter, nutmeg and salt & pepper.Using a potato masher, mash until smooth.Transfer the sweet potato mixture to a baking dish.Preheat your oven to 375 degrees.Sprinkle the top of the casserole with the marshmallows, then bake until the casserole is warmed through and marshmallows are lightly browned, about 15 to 20 minutes.