Heat oven to 375° F. Pierce the sweet potatoes several times with a fork and place on a foil-lined baking sheet. Bake until softened, 1 to 1 1/2 hours, depending on size; let cool. Slice each sweet potato in half lengthwise. Scoop the flesh into a saucepan and discard the skins. Add the maple syrup (to taste), butter, and sour cream. Using a wooden spoon or potato masher, mix until smooth. Season with the salt and nutmeg. Place over medium heat, stirring occasionally, until warmed through, about 5 minutes.Make Ahead: Assemble the puree the day before (but don't heat it). Allow an extra 5 minutes for warming.