Preheat the oven to 375°F.
Grease a 9x13-inch baking dish with cooking spray or unsalted butter.
Spiralize the sweet potatoes and add them to a large bowl. Pour the melted butter over the sweet potatoes then add the salt, cinnamon and nutmeg. Gently toss the sweet potatoes until they are evenly coated in the butter and spices. Transfer the mixture to the baking dish, pour the water over the top of the sweet potatoes, and cover with foil. Bake for 20 to 25 minutes, or until the sweet potatoes are tender. Remove the casserole from the oven then prepare the topping by combining the pecans, flour, sugar and salt in a small bowl. Cube the butter and add it to the bowl, working it into the flour mixture until it’s well incorporated.
Add the pecan topping and the marshmallows to the casserole, evenly dispersing both over the top of the sweet potatoes. Return the casserole to the oven and bake it for an additional 15 minutes, or until the marshmallows are browned and gooey, and the sweet potatoes are soft and tender. Remove and serve warm.
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