Preheat the oven to 375°F.

Spray a 9-inch square baking dish liberally with nonstick cooking spray.

Peel the sweet potatoes and slice uniformly about 1/8-inch thick. Arrange the slices in the pan in rows, slightly overlapping one another. Sprinkle salt and black pepper over the sweet potatoes.

Pour the heavy cream into a large measuring cup. Add the thyme leaves and nutmeg. Whisk to combine. Pour the mixture over the sweet potatoes.

Sprinkle the grated Parmesan and Gruyere cheeses in an even layer over the sweet potatoes.

Bake the gratin on middle rack of oven until bubbling and browned on top, 40-45 minutes.

Check to see that gratin is done. Remove from oven or add time as needed.

Allow the dish to cool for 10 minutes. Plate and serve.