Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.

In a medium saucepan, combine heavy cream and cornstarch over medium-low heat, whisk until smooth. Add softened butter, minced garlic, fresh thyme, kosher salt, and pepper, stirring until the butter melts.

Peel and thinly slice the sweet potatoes to about 1/8 inch thick.

Layer the sweet potato slices in the prepared baking dish, seasoning with salt and pepper between layers.

Pour the garlic butter sauce evenly over the layered sweet potatoes.

Cover the baking dish tightly with aluminum foil and bake for 30 minutes.

Remove the foil and sprinkle Parmesan cheese on top. Return to the oven uncovered and bake for an additional 10 minutes.

Broil for 1-2 minutes at the end for a golden crust.

Let the dish rest for 10 minutes before serving.