This classic French potato gratin is all about technique and patience. The key is to slice the potatoes uniformly thin and to layer them carefully with the cream mixture and cheese. Take your time with each step, and you'll be rewarded with a perfectly creamy, golden gratin that's worth the effort.

1. Step 1: Preheat oven and prepare baking dish

   Preheat your oven to 350°F (175°C). Generously butter a 9x13-inch baking dish.

2. Step 2: Prepare the cream mixture

   In a large saucepan, combine the heavy cream, milk, minced garlic, thyme, nutmeg, salt, and pepper. Bring to a gentle simmer over medium heat, stirring occasionally. Once simmering, remove from heat and set aside.

3. Step 3: Slice the potatoes

   Peel the potatoes and slice them into thin, uniform rounds about 1/8-inch thick. A mandoline slicer works best for this, but a sharp knife will also work.

4. Step 4: Create the first layer

   Layer one-third of the potato slices in the buttered baking dish, slightly overlapping them. Pour about one-third of the cream mixture over the potatoes, then sprinkle with one-third of the Gruyère cheese.

5. Step 5: Complete the layers

   Repeat the layering process two more times with the remaining potatoes, cream mixture, and Gruyère cheese, creating three layers total.

6. Step 6: Add Parmesan topping

   Sprinkle the grated Parmesan cheese evenly over the top layer.

7. Step 7: Initial baking period

   Cover the baking dish with aluminum foil and bake for 45 minutes.

8. Step 8: Final browning

   Remove the foil and continue baking for an additional 15-20 minutes, or until the top is golden brown and bubbly, and the potatoes are tender when pierced with a knife.

9. Step 9: Rest before serving

   Remove from the oven and let the gratin rest for 10-15 minutes before serving. This allows the layers to set and makes it easier to serve.

For the best results, use starchy Yukon Gold potatoes which hold their shape while becoming creamy when baked. This gratin pairs beautifully with roasted meats, particularly beef or lamb, and can be prepared up to a day in advance and reheated before serving.