Poke the unpeeled sweet potatoes with a sharp knife in the center. Boil whole in a large saucepan or stockpot covered with water and salt until tender, about 25-30 minutes.
Stir together the melted butter, brown sugar, maple syrup and Grand Marnier.
Preheat the oven to 375 degrees. Oil the casserole dish.
Remove the sweet potatoes when they are tender to a plate to cool. Peel the sweet potatoes. Slice into ¼" thick slices.
Layer the sweet potato slices with the butter sugar mixture, dividing between the layers and reserving enough for the top layer. Bake for 15 minutes.
Remove from the oven. Add mini marshmallows and pecans interspersed on top. Drizzle with remaining 2 tablespoons of maple syrup. Bake 10 minutes more.
Increase oven temperature to 425. Bake 4-5 minutes until marshmallows are light golden brown. Add reserved chopped pecans and a sprinkle more of cinnamon and nutmeg.