A good potato gratin is all about balance. You want the potatoes cooked but not too cooked and the dish slightly saucy—not soupy or dry. The most important part, however, is the crusty, nicely browned top, which is what makes it a gratin.
Game plan: This gratin can be completely made ahead. Cool to room temperature, cover with plastic wrap or aluminum foil, and refrigerate until ready to serve. Rewarm uncovered in a 350°F oven for about 30 to 35 minutes.
2 poundsrusset potatoes, peeled and cut into 1/8-inch-thick slices
1 cupheavy cream
1 cup whole milk
2 teaspoonskosher salt
1/2 teaspoon freshly ground black pepper
1/8 teaspoon freshly grated nutmeg
4 ounceschèvre (fresh goat cheese), crumbled
1 tablespoon unsalted butter, cut into 6 pieces, plus more for coating the dish
Heat the oven to 350°F and arrange a rack in the upper third. Coat an 8-by-8-inch baking dish with butter; set aside.
Place the potatoes, cream, milk, salt, pepper, and nutmeg in a large saucepan over medium heat and bring to a gentle boil. While the mixture heats up, gently fold the potatoes from time to time. (It should take about 15 minutes for the potato mixture to reach a gentle boil.) Remove the saucepan from heat.
Using a slotted spoon, transfer half of the potatoes to the prepared baking dish and arrange them in an even layer. Sprinkle with half of the chèvre. Repeat with the remaining potatoes and chèvre. Slowly pour all of the warm cream mixture over the potatoes. Scatter the butter pieces on top.
Bake until the potatoes are tender when pierced with the tip of a knife and the top is golden brown and bubbly, about 45 to 50 minutes. Remove from the oven and let sit for at least 10 minutes before serving.