Unsalted butter, for greasing the baking dish
4 large (about 2 pounds) Yukon gold potatoes, peeled and sliced 1/8-inch thick
1 1 ½-pound butternut squash, peeled, seeded, and sliced 1/8-inch thick
Freshly ground black pepper
Generous pinch nutmeg
1 clove garlic, grated
1 ¾ cups heavy cream
1 cup shredded Gruyère
Preheat the oven to 325ºF. Generously butter a 12-inch oval baking dish. Season the potato and butternut squash slices to taste with salt and pepper. Layer them into the dish, arranging them in neat, overlapping circles and alternating layers of potato and squash. Stir the nutmeg and garlic into the cream and slowly pour over the potatoes and squash. Sprinkle evenly with the Gruyère. Bake for 1 ¼ to 1 ½ hours, or until the top is golden brown and the potatoes and squash are tender. Cover with foil to keep warm and allow to rest 20 to 25 minutes before serving.
Serves 6. Make this gratin in the fall, when butternut squash is abundant and rich food is welcome. Yukon gold potatoes will become creamy but retain their texture after cooking. A mandoline makes easy work of slicing potatoes, but I prefer to slice the squash with a chef’s knife. Season the gratin carefully before it goes into the oven because it’s hard to add salt and pepper once it is cooked. I toss the potato and squash slices with salt and pepper in a large bowls and taste a bit of each raw to check the amount of salt—they should taste slightly salty at this point for a perfect finished gratin—and spit it out. Minced fresh thyme or sage can be added along with the salt and pepper. Letting the gratin rest before serving makes it possible to cut neat portions.