Wash, peel and cut the sweet potatoes and then add them to the Instant Pot.
Add 1 cup of water, close the pot and set to Manual / Pressure Cook, High Pressure for 4 minutes.
Once cooking has completed, do a QPR (Quick Pressure Release) and open the pot once all the pressure has been released.
Mash the potatoes and then add the milk, whipping cream and melted butter and mix in well.
Next add the brown sugar and spices and mix in well. Once the spices have been thoroughly mixed in, add the walnuts and pineapple and mix into the casserole.
Spoon the mix into a baking dish and then place in a 375° F oven for 20-30 minutes. This will thicken and darken the casserole. The longer you leave the casserole in the oven, the thicker and darker it will become.
Remove the casserole from the oven and fluff with a fork to release the steam so that the casserole will continue to thicken and cool.
make the Marshmallow Topping
To get the perfect marshmallow and cherry topping, let the casserole cool for 10-15 minutes after taking it out of the oven.
Once cooled, cover the top of the casserole with a layer of marshmallows and then add cherries throughout.
Turn the oven to the Low Broil setting. Place the baking dish back into the oven on the middle rack and broil for approximately 3-5 minutes.
Keep a constant eye on the casserole! The marshmallows will brown quickly and a little distraction could lead to burned marshmallows! As soon as the marshmallows start turning brown take the casserole out of the oven and enjoy!!