Step 1: Prepare the Potatoes and Dish

1. Preheat your oven to 375°F (190°C).
2. Butter a 9×13-inch baking dish generously to prevent sticking.
3. Wash, peel, and slice the potatoes into thin, uniform slices (about 1/8-inch thick). Use a mandoline slicer for consistent results.

Step 2: Make the Cream Sauce

1. In a medium saucepan over medium heat, melt 2 tablespoons of butter.
2. Add the minced garlic and sauté for 1-2 minutes until fragrant.
3. Pour in the heavy cream and milk, stirring constantly.
4. Add the thyme, nutmeg (if using), and season with salt and pepper to taste. Bring the mixture to a gentle simmer, then remove from heat.

Step 3: Assemble the Layers

1. Arrange a single layer of potato slices in the prepared baking dish.
2. Pour a portion of the cream sauce over the potatoes, ensuring even coverage.
3. Sprinkle a mixture of cheddar and Gruyère cheese over the layer.
4. Repeat the process (potatoes, cream, cheese) until all ingredients are used, finishing with a generous layer of cheese on top.

Step 4: Bake the Dish

1. Cover the dish tightly with aluminum foil and bake for 30 minutes.
2. Remove the foil and continue baking for another 25-30 minutes, or until the potatoes are tender and the top is golden and bubbly.

Step 5: Rest and Serve

1. Let the dish rest for 10 minutes before serving to allow the layers to set.
2. Garnish with chopped parsley or thyme for added flavor and color.