Using a mandolin, slice the peeled sweet potatoes into very thin, uniform slices. Arrange the potato slices vertically in the dish, filling the dish, without crowding it. Make sure there’s space in between each slice so it cooks evenly.
In a small bowl, whisk together the melted butter, brown sugar, maple syrup, rum, cinnamon, ginger, nutmeg and salt. Pour the sauce on top of the sweet potatoes, making sure each slice is evenly coated.
Cover the baking dish tightly with aluminum foil and bake at 400° for 1 hour. Remove the aluminum foil and cook for another 15-20 minutes until the potatoes are slightly browned, caramelized and fork tender. Serve and enjoy!