- 1
Adjust oven rack to upper-middle position and heat oven to 400 degrees. Grease 8½ by 5½-inch baking dish. Place potato slices in medium bowl.
- 2
Melt butter in small saucepan over medium heat. Add shallot and salt and cook until softened, about 2 minutes. Stir in garlic, thyme, pepper, nutmeg, and cayenne and cook until fragrant, about 30 seconds. Stir in flour and cook until incorporated, about 10 seconds. Whisk in cream, bring to simmer, and cook until beginning to thicken, about 30 seconds.
Timer: 2m - 3
Pour sauce over potato slices and toss to coat thoroughly. Transfer potato mixture to prepared dish and gently press potato slices into even layer, removing any air pockets. Cover dish tightly with aluminum foil and bake until potatoes are almost tender, 35 to 40 minutes.
Timer: 40m - 4
Carefully remove foil and sprinkle with Gruyère. Continue to bake, uncovered, until cheese is lightly browned and potatoes are tender, about 10 minutes. Transfer gratin to wire rack and let cool for 5 to 10 minutes before serving.
Timer: 10m