Get a pot of cold water and put the sweet potatoes in there. Bring to a boil and then reduce the heat and simmer until the potatoes are tender. Should be about 20 minutes.
Drain the potatoes and put them in an ice bath. When cooled, the skins should be peelable. Peel all the skins and set potatoes aside.
In a small bowl, blend together the pecans, brown sugar, ginger, cinnamon, nutmeg, and butter, using your fingers.
Heat oven to 450 and butter a baking dish. While oven is heating, slice the sweet potatoes about 1/4 inch thick.
Layer the bottom of the dish with a third of the sweet potatoes, and dab a third of the awesome mixture on top. Repeat with the remaining potatoes and mixture till you run out.
Cover with foil, and bake for about 40 minutes.
Remove from oven and turn it up to broil. Remove the foil and try hard not to pass out from the overwhelmingly amazing smell. Top the gratin with marshmallows and heat it, uncovered, until browned. About a minute.