1. Preheat your oven to 350°F (175°C). Butter a 9×13-inch baking dish generously with unsalted butter to prevent sticking and add richness.
2. In a medium saucepan, combine the heavy cream, whole milk, minced garlic, salt, pepper, and nutmeg. Warm over medium heat until just about to simmer, stirring occasionally. Remove from heat and set aside.
3. Arrange a layer of thinly sliced potatoes in the bottom of the prepared baking dish, overlapping them slightly—this layering is what creates that beautiful texture.
4. Pour a ladle of the warm cream mixture over the potato layer, then sprinkle a handful of Gruyère and Parmesan cheeses evenly on top.
5. Repeat layering potatoes, cream, and cheese until all ingredients are used, finishing with a generous layer of cheese on top. Dot the surface with the remaining butter pieces for a golden crust.
6. Cover the dish tightly with aluminum foil and bake for 45 minutes to 1 hour, or until the potatoes are tender when pierced with a fork.
7. Remove the foil and increase oven temperature to 400°F (200°C). Bake for an additional 15-20 minutes, or until the top is bubbly and golden brown.
8. Let the gratin rest for 10 minutes before serving. This helps the layers set for perfect slicing.

One of my favorite memories is when Ouma and I would sneak into the kitchen while Mama Lou was baking this dish, peeking through the oven window to watch the cheese bubble and brown. I always remind myself to let it rest properly; rushing this step was a mistake I made once, and the layers just didn’t hold together as beautifully.