1. Begin by peeling and cutting the sweet potatoes into 1-inch pieces. Place them in a large pot and cover with water by 2 inches. Bring the pot to a boil over high heat and cook the sweet potatoes until they are fork-tender, about 10 minutes. Drain the water from the pot and return the cooked sweet potatoes to the pot.

2. Using a potato masher or an electric mixer, mash the sweet potatoes until they are mostly smooth. Then, add the unsalted butter, packed light brown sugar, heavy cream, kosher salt, vanilla extract, ground cinnamon, and ground nutmeg. Mix all the ingredients together until well combined and the mixture is creamy and cohesive.

3. Preheat your oven to 350°F. Grease the casserole dish of your choice and spoon the sweet potato mixture into the dish, smoothing the top with the back of a wooden spoon or a mixing spatula. Sprinkle the mini marshmallows evenly over the top of the sweet potato mixture.

4. In a medium bowl, combine the all-purpose flour, brown sugar, salt, old-fashioned rolled oats, and ground cinnamon. Cut in the room temperature butter using a pastry cutter or your fingers until the mixture is crumbly. Sprinkle the topping over the marshmallows, covering the entire casserole.

5. Place the casserole dish in the preheated oven and bake for 20 to 30 minutes, or until the topping is crisp and the marshmallows are golden brown. Keep a close eye on the casserole to prevent the topping from burning.