Prepare the Potatoes - Wash and dry sweet potatoes, then individually wrap them in foil. Place on a sheet pan lined with foil. Transfer to the center position of a cold oven.
Bake the Potatoes - Heat the oven to 375ºF (191ºC). Bake for 1 hour, then flip over to cook evenly. Bake until they can be easily pierced in the center with a knife, about 15 to 30 minutes.
Make the Potato Puree - Cool the sweet potatoes until they are easy to handle. Peel and transfer to a food processor or blender. Puree on high speed until relatively smooth, scraping the container as needed, about 1 minute. Alternatively, crush it with a potato masher for a chunkier texture.
Add the Seasonings - Add heavy cream, vanilla extract, ½ teaspoon cinnamon, and nutmeg to the food processor. Use a spoon to mix the ingredients, allowing the cream to move to the container's bottom. Process on low speed until smooth, about 1 to 2 minutes.
Place in Casserole Dish - Transfer the whipped potatoes into an 11x8-inch or 9x9-inch casserole dish and spread out evenly. Reduce oven temperature to 350°F (177ºC).
Make Candied Nut Topping - In a medium bowl, combine chopped pecans, crispy rice cereal, ½ teaspoon cinnamon, ¼ teaspoon salt, and brown sugar. Add melted butter and stir to combine.
Bake the Topping - Evenly spread the topping over the sweet potatoes. Place the casserole dish on a baking sheet and bake until golden brown and bubbling, about 25 to 30 minutes.
Toast the Marshmallows - Remove the casserole from the oven. Sprinkle the marshmallows on top. Bake until the surface is lightly browned and slightly melted, about 5 to 10 minutes. Serve warm.
Notes
Make-Ahead: The baked sweet potatoes and the dried nut mix can be stored in an airtight container for up to 3 days. Bake the pecan topping and marshmallows the day of serving.
Storing and Reheating: The casserole can be cooled completely, covered, and refrigerated for up to 5 days. Reheat small portions in the microwave until warmed through.
Make It Dairy Free: Substitute heavy cream for canned coconut cream or coconut milk. Substitute butter for coconut oil, margarine, or plant-based butter.