Equipment Needed
- Mandoline slicer or a very sharp knife and a steady hand
- 9×13 inch or 3-quart baking dish
- Medium saucepan
- Whisk
- Large mixing bowl
- Box grater
Step-by-Step Instructions
My process begins with the potatoes. Peeling and slicing them uniformly thin is the most critical step for even cooking and elegant layers. I use a mandoline slicer set to 1/8-inch thickness—it’s a game-changer for speed and consistency, but a sharp knife and patience work too. As I slice, I immediately place the potatoes in a large bowl of cold water. This prevents them from browning and, more importantly, rinses off excess starch, which can make the sauce gluey. I learned this the hard way when my first batch had a cloudy, thick sauce instead of a silky one. I let them soak for about 15 minutes while I prepare the cream mixture.
While the potatoes soak, I build the flavorful cream bath. In my medium saucepan, I combine the heavy cream, whole milk, minced garlic, thyme, nutmeg, salt, and white pepper. I bring this mixture just to a simmer over medium heat, then immediately remove it from the burner. The goal is to infuse the cream with all those beautiful flavors without boiling it, which could cause it to separate or form a skin. The aroma at this stage is heavenly—garlicky, herbal, and rich. I let this infused cream cool for about 10 minutes; pouring boiling cream over raw potatoes can start cooking them unevenly.
Now, for the assembly, which is my favorite part. I drain my potato slices and pat them very dry with clean kitchen towels. Any residual water will dilute the sauce. I generously butter my baking dish. Then, I create a single, overlapping layer of potato slices on the bottom, like shingling a roof. I sprinkle this layer lightly with a portion of the shredded Gruyère and a little Parmesan. I repeat this process, creating 3-4 layers, ending with just potatoes on top. Once all my layers are built, I slowly pour the warm, infused cream mixture over the entire dish, letting it seep down through all the layers. I use the back of a spoon to press down gently, ensuring the cream reaches the bottom.(See the next page below to continue…)