Spray 9x13 baking dish with non-stick cooking spray, set aside.
Peel sweet potatoes, cut sweet potatoes into 2-inch pieces. Place in a large pot and add water to cover. Bring to a boil over medium high heat until fork tender, about 20 minutes.
Remove from heat, drain well. Place sweet potatoes in a large mixing bowl and mash until creamy. You can also use a hand mixer if you prefer whipped potatoes.
Zest orange and juice orange in a small bowl, you'll need about ¼ cup juice. Whisk with melted butter.
In a small bowl, brown sugar, cinnamon, nutmeg and salt.
Layer half of the sweet potato filling in the prepared baking dish. Sprinkle with half of the brown sugar mixture, and drizzle with half of the butter. Top with half of the toasted pecans. Repeat layers with the remaining ingredients.
Top the entire casserole evenly with marshmallows.
To make ahead, cover with plastic wrap and refrigerate overnight or up to two days.
The next day, remove it from the refrigerator and set on the counter for 30 minutes prior to baking.
Preheat oven to 350 degrees. Bake for 25-30 minutes or until marshmallows are golden and casserole is bubbling and hot. Enjoy!