• Wash and peel sweet potatoes. Chop the potatoes into roughly equal-sized pieces.
• Bring a large pot of water to boiling. Add the sweet potatoes and boil until soft (15-20 minutes).
• Drain the sweet potatoes and add to the bowl of an electric mixer. Add the cinnamon, nutmeg, brown sugar, butter, and milk.
• Start mixer on low, and gradually increase speed to medium. Mix until sweet potatoes are mashed and all <g class="gr_ gr_127 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="127" data-gr-id="127">intgredients</g> are incorporated well.
• Lightly spray a 9x13 pan with cooking spray. Transfer mashed sweet potatoes to the pan and spread evenly.
• Cover with aluminum foil and bake at 350 degrees for 30 minutes.

For the topping:

• While the sweet potatoes are baking, mix the topping.
• In a medium bowl, combine brown sugar, cinnamon, nutmeg, flour, and pecans. Stir well.
• Add melted butter and stir well to combine.
• After 30 minutes, remove the sweet potato casserole from the oven.
• Remove the aluminum foil.
• Sprinkle the brown sugar topping over the sweet potato casserole.
• Evenly sprinkle the marshmallows over the brown sugar topping.
• Return the sweet potato casserole to the oven (uncovered), and bake an additional 15-20 minutes, until the toppings are browned.
• Let sit for 5 minutes before serving.