01 -
Preheat oven to 375°F. Grease a 9×13-inch baking dish with butter or nonstick spray.
02 -
In a small saucepan, whisk cornstarch into heavy cream until smooth. Add butter, minced garlic, and chopped thyme. Season with kosher salt and black pepper. Heat over medium-low, stirring constantly, until butter melts and mixture is hot but not boiling.
03 -
Arrange the sweet potato slices evenly in the prepared baking dish, overlapping slightly as needed.
04 -
Pour the hot garlic cream mixture evenly over the arranged sweet potatoes. Cover the dish with foil and bake for 30 minutes.
05 -
Remove foil and sprinkle grated Parmesan cheese on top. Return to oven and bake for an additional 10 minutes, or broil for a golden brown finish, watching closely to prevent burning.
06 -
Let the bake stand for 10 minutes after removing from the oven before serving to allow the sauce to thicken.