Roast or boil the sweet potatoes until fully cooked. Set aside to cool before peeling.
Preheat the oven to 375°F and grease a 9 x 13″ pan and set aside.
Combine the cooked and peeled sweet potatoes with the brown sugar, melted butter, cinnamon, ginger, nutmeg, and salt in a large bowl. Use a potato masher or a large fork to combine until homogenous.
Place the sweet potato mixture in prepared pan.
Combine all topping ingredients except for the marshmallows for the crumble in a large bowl and use a fork to combine until the mixture resembles wet sand and starts to clump.
Sprinkle the crumble topping over the sweet potato mixture. Place in the oven for 15 minutes.
After 15 minutes, remove from the oven and top with the full bag of marshmallows. Bake for 15 more minutes until marshmallows are golden brown and toasty. Allow to cool for 15 minutes and then serve right away! Enjoy!