Set your oven to preheat at 375°F (190°C). Generously butter a 9x13-inch baking dish to ensure the potatoes don’t stick and the gratin slides out smoothly.

Start by arranging a single layer of thinly sliced potatoes across the base of the prepared dish. Slightly overlap each slice for even coverage. Lightly season with salt and freshly ground black pepper to enhance flavor.

In a medium saucepan, combine the heavy cream, whole milk, minced garlic, and nutmeg. Warm over medium heat until the mixture is steaming but not boiling. Remove from the heat and stir in half of the chopped thyme for an aromatic infusion.

Evenly scatter one-third of both the Gruyère and Parmesan cheeses over the seasoned potato layer. Pour one-third of the warm cream mixture over the top, allowing it to seep between the slices.

Continue layering potatoes, seasoning, cheeses, and cream mixture two more times, ending with a final pour of the cream mixture and a generous topping of cheese to create a bubbling golden crust as it bakes.

Cover the baking dish with foil and bake in the preheated oven for 40 minutes. Then, remove the foil and continue baking for an additional 20 to 30 minutes. The gratin is ready when the top is deeply golden, the cheese is bubbling, and a knife slides easily through the tender potatoes.

Sprinkle the remaining thyme leaves over the top just before serving to add a fresh, herby finish. Allow the gratin to rest for 10 minutes so the layers set and flavors meld before slicing and serving.