Step 1: Preheat the Oven and Prepare the Dish

1. Preheat your oven to 375°F (190°C).
2. Grease a 9×13-inch baking dish with unsalted butter to prevent sticking and enhance flavor.

Step 2: Prepare the Potatoes

1. Peel the potatoes and slice them thinly (about 1/8 inch thick) using a sharp knife or mandoline slicer for uniformity.
2. Place the sliced potatoes in a bowl of cold water to prevent them from browning. Drain and pat them dry with a clean kitchen towel before layering.

Step 3: Make the Cream Mixture

1. In a medium saucepan, combine heavy cream, whole milk, minced garlic, nutmeg, salt, and black pepper.
2. Heat over medium-low heat, stirring occasionally, until the mixture is warm but not boiling. This step infuses the cream with garlic and spices.

Step 4: Assemble the Gratin

1. Arrange half of the potato slices in an even layer in the prepared baking dish.
2. Pour half of the warm cream mixture over the potatoes.
3. Sprinkle half of the Gruyere and Parmesan cheese over the top.
4. Repeat with the remaining potatoes, cream mixture, and cheese, finishing with a generous layer of cheese on top.

Step 5: Bake the Gratin

1. Cover the dish with aluminum foil and bake in the preheated oven for 50 minutes.
2. Remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork.
3. Let the gratin rest for 10 minutes before serving to allow the sauce to thicken.

Step 6: Garnish and Serve

1. Garnish with fresh thyme sprigs or chopped parsley for a pop of color and added flavor.
2. Serve hot as a side dish or a main course.