• Preheat oven to 350 degrees. In a medium baking dish, spray with non-stick cooking spray. Set aside. (An oval or rectangular baking dish is ideal (about 9 - 10").
• Wash, peel, and cut the sweet potatoes into 1" pieces. Place in a large saucepan and cover with cold water by 1 inch. Bring to a boil over medium-high heat, then reduce to a simmer and cook until fork-tender, 14–18 minutes. A piece should break apart easily when pierced with a fork.
• Drain well. Return the potatoes to the warm pan and let them steam dry for 2–3 minutes.
• Add the brown sugar, half and half, butter, egg, vanilla, and salt. Mash with a potato masher until mostly smooth. Small lumps are okay.
• Spread the mixture evenly in the prepared dish. Bake uncovered for 35–40 minutes, until the edges are set and the center no longer looks wet.

Pecan Topping:

• While the casserole bakes, prepare the pecan topping. In a mixing bowl, combine the brown sugar, pecans, flour, cinnamon and nutmeg. Stir together to combine. Pour the melted butter into the brown sugar mixture, and stir until crumbly.
• Remove the casserole from the oven. Sprinkle with half the pecan crumble, then the mini marshmallows, then the remaining crumble.
• Bake for another 10-15 minutes at 350 degrees to allow the pecans to toast and the marshmallows to lightly brown. Check at 10 minutes.
• Let the casserole rest at least 10 minutes before serving.