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

In a small saucepan over medium-low heat, whisk together cornstarch and heavy cream until smooth. Slowly add butter, garlic, thyme, salt, and pepper until just hot.

Thinly slice the sweet potatoes to about 1/8 inch thick and arrange them in the prepared baking dish, overlapping slightly. Sprinkle salt and pepper between layers.

Pour the hot cream mixture evenly over the sweet potato layers and press down with a spatula to submerge.

Bake uncovered for 30 minutes, until sweet potatoes are fork-tender.

Sprinkle grated Parmesan cheese over the top and bake for another 10 minutes until the cheese is melted and golden brown.

For extra crispiness, turn on the broiler for the last 1-2 minutes of baking.

Let the dish rest for 10 minutes before serving warm.