Place sweet potatoes in a large saucepan of cold water, and bring to a boil. Reduce heat and simmer until potatoes are just tender- 20 minutes. Drain. Place potatoes in a bowl of cold water until skins begin to loosen. Peel cooled potatoes- set aside.
In a small bowl, combine pecans, sugar, ginger, cinnamon, nutmeg and butter- using your fingers to blend. Slice reserved potatoes 1/4" thick.
Preheat oven to 450°. Butter an 8x8 baking dish. Layer a third of the potatoes on the bottom. Sprinkle with a third of the nut mixture. Repeat 2 more times. Cover with foil and bake until heated through- about 40 minutes.
Remove dish from oven and raise temperature to broil. Top gratin with a layer of marshmallows and heat until just browned- 30-60 seconds. Serve hot. Serves 6