How to Make It
Preheat oven to 350°F. Layer sweet potato slices in a lightly greased 13- x 9-inch baking dish.
Melt butter in a small saucepan over medium. Add sugars, stirring until well combined. Stir in cream; cook, stirring often, just until mixture comes to a simmer, 5 to 7 minutes. Remove from heat; stir in vanilla extract, cinnamon, salt, and nutmeg.
Pour sugar mixture evenly over sweet potatoes, and cover with lightly greased aluminum foil.
Bake, covered, in preheated oven, about 40 minutes. Uncover and gently stir potato mixture. Bake, uncovered, until potatoes are tender, 25 to 30 more minutes. Transfer potatoes to a serving bowl with a slotted spoon; pour syrup over potatoes. Serve immediately.
Make It Ahead
Assemble up to 3 days in advance. Make the recipe through Step 3, store it covered in the refrigerator, and then bake it on Thanksgiving Day.