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.
Heat oven to 375ºF (191ºC). Bake for 1 hour, then flip over to evenly cook. Continue to bake until they can be easily pierced in the center with a knife, about 15 to 30 minutes.
Cool 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 with a potato masher for a chunkier texture.
Add heavy cream, vanilla extract, ½ teaspoon cinnamon, and nutmeg to the food processor. Use a spoon to mix the ingredients to allow the cream to move to the container's bottom. On low speed, process the mixture until smooth, about 1 to 2 minutes.
Transfer the whipped potatoes into an 11-by-8-inch or 9-by-9-inch casserole dish and spread out evenly. Reduce oven temperature to 350°F (177ºC).
In a medium bowl combine chopped pecans, crispy rice cereal, ½ teaspoon cinnamon, ¼ teaspoon salt, and ⅔ cup brown sugar. Add ¼ cup melted butter and stir to combine.
Evenly spread the topping over the sweet potatoes. Place the casserole dish on a baking sheet and bake until the topping is golden brown and bubbling, about 25 to 30 minutes.
Remove the casserole from the oven. Evenly sprinkle 2 cups of mini-marshmallows on top and bake until the surface is lightly browned and slightly melted, about 5 to 10 minutes. Serve warm.
Make-ahead: The baked sweet potatoes and the dried nut mix can be stored in an airtight container up to 3 days in advance. 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.