Preheat the oven to 400°F. Pierce the potatoes all over with a fork and place on a foil or parchment lined rimmed baking sheet. Bake until they are tender on the inside, about 45 minutes. Reduce oven temperature to 350°F. Let the potatoes cool slightly then slice open and scoop the flesh into a large bowl, discarding the skins. Mash with a potato masher, being careful not to over-mash. Add the milk, maple syrup, eggs, vanilla, salt, and nutmeg and stir until just combined.

In a small bowl combine the flour and butter, cutting the butter into the flour with the back of a fork. Add the brown sugar and pecans and stir until combined.

Spread the potato mixture into a 13 by 9-inch or similarly sized baking dish. Sprinkle the pecan mixture in diagonal rows over the sweet potatoes, leaving a 1 1/2-inch gap between rows. Bake at 350°F oven until golden brown on top, about 30 minutes. Remove from oven and gently place the marshmallows between the rows of the pecan mixture. Bake for an additional 10 minutes, or until the marshmallows are just golden brown. Serve.