Peel and cut the sweet potatoes into large chunks and place in a large sauce pan. Cover with water and bring to a boil
Boil until the sweet potatoes are fork tender and easily pierced with a fork. They should fall apart easily
Drain the sweet potatoes and add them back to the sauce pan. Mash with a potato masher. Add the rest of the sweet potato filling ingredients: butter, brown sugar, salt, cinnamon, vanilla extract, and nutmeg and mix until well combined
In a separate bowl, make the pecan topping. Stir together the melted butter, brown sugar, all purpose flour, salt, and pecan pieces until combined
Grease a 9 by 13 baking dish with cooking spray
Add the sweet potato filling and spread out evenly
Spread the pecan topping evenly over the top of the sweet potatoes
Add the mini marshmallows over the top in an even layer
Bake the sweet potato casserole at 350 degrees for 25-35 minutes, or until bubbly and the marshmallows are golden brown on top. Watch closely towards the end to get the perfect golden brown without burning