Preheat your oven to 375°F (190°C) and grease the baking dish with butter or non-stick spray.

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

Peel and slice sweet potatoes into thin rounds and layer in the greased dish, seasoning each layer with salt and pepper.

Layer the sliced sweet potatoes tightly in the baking dish, seasoning as you go.

Pour the warm cream mixture over the layered sweet potatoes, ensuring they are evenly covered.

Bake uncovered in the preheated oven for about 30 minutes.

Sprinkle a layer of Parmesan cheese on top and return to the oven for an additional 10 minutes.

Switch to broil and cook for an additional 1-2 minutes until cheese is golden brown.

Let the dish rest for about 10 minutes before serving.