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

In a small saucepan, whisk together the heavy cream and cornstarch over medium-low heat until smooth. Add butter, garlic, thyme, salt, and pepper. Heat until warmed through, about 5 minutes.

Slice the sweet potatoes into 1/8-inch thick rounds and arrange in the baking dish, overlapping slightly.

Pour the creamy sauce evenly over the sweet potato layers, ensuring all slices are coated.

Bake for 30 minutes until tender and caramelized at the edges.

Sprinkle Parmesan cheese on top and return to the oven for another 10 minutes until melted and bubbly.

Broil for 1-2 minutes for a crispy top, watching closely to avoid burning.

Let it rest for about 10 minutes before serving.