Follow these steps to create your amazing Potato Gratin:

1. Preheat your oven: Preheat to 375°F (190°C). Grease a 3-quart baking dish with unsalted butter.

2. Prep the potatoes: Peel and slice the Yukon Gold potatoes into thin rounds, approximately 1/8 inch thick. A mandoline can be helpful for uniform slices.

3. Make the cream mixture: In a mixing bowl, combine the heavy cream, whole milk, minced garlic, chopped thyme, salt, and black pepper.

4. Layer the potatoes: Place one layer of sliced potatoes in the greased baking dish, slightly overlapping each slice. Pour a bit of the cream mixture over them, followed by a sprinkle of Gruyère and Parmesan cheese.

5. Repeat layers: Continue layering potatoes, cream, and cheese until the dish is full. Reserve some cheese for the top layer.

6. Finish with cheese: Pour any remaining cream mixture over the final layer of potatoes. Top with the remaining cheeses for a golden crust.

7. Bake: Cover the dish with aluminum foil and place it in the preheated oven. Bake for 45 minutes.

8. Remove foil: After 45 minutes, carefully remove the foil to allow the cheese to brown. Bake for an additional 15-20 minutes or until the top is bubbly and golden brown.

9. Cool: Once baked, remove the dish from the oven and let it rest for about 10 minutes to set before serving.

10. Garnish: Optionally, sprinkle with additional fresh thyme or parsley for a touch of color.

These simple yet effective steps will lead you to create an incredible Potato Gratin that will impress anyone who tastes it!