Cook the sweet potatoes until soft and mushy.
Preheat oven to 350°F.Spray a pan with nonstick cooking spray. Add sweet potatoes, half and half, maple syrup, vanilla, cinnamon, and nutmeg to a stand mixer bowl. Beat on medium speed for about 2 minutes or until well combined and mostly smooth. Spread mixture evenly into the baking dish using a spatula. In a small bowl, cut the butter and flour together using a pastry cutter or 2 forks until the butter is about pea-sized. Mix in brown sugar, salt, and pecans. Sprinkle mixture evenly over sweet potatoes. Bake 20 minutes. Scatter the marshmallows on top then continue to bake until the marshmallows are puffy and golden (about 10-12 minutes). Cool slightly, serve.
Store in an airtight container in the refrigerator for 4-5 days.