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, and cook, stirring often, just until mixture comes to a simmer, about 5 to 7 minutes. Remove from heat, and stir in orange zest, rosemary, and salt.
Pour sugar mixture evenly over potatoes, and cover with lightly greased aluminum foil.
Bake, covered, in preheated oven, about 40 minutes. Uncover and gently stir potato mixture to ensure even cooking. Bake, uncovered, until potatoes are tender, 25 to 30 more minutes. Transfer potatoes to a serving bowl with a slotted spoon, and pour syrup over potatoes. Top with additional orange zest and rosemary sprigs. Serve immediately.
Prepare recipe through Step 2. Lightly coat potatoes with cooking spray, and cover tightly with plastic wrap. Transfer syrup to an airtight container, and refrigerate syrup and potatoes separately. You can make only the syrup ahead, and cut the potatoes the day you cook them to ensure that they don’t oxidize. Combine syrup and potatoes in a 13-x 9-inch baking dish on the day off. Proceed with recipe as directed.