Preheat your oven to 375°F (190°C). Slice the potatoes thinly, about ⅛ inch thick.

Melt the butter in a medium saucepan over medium heat. Add onion, sauté for 5 minutes until translucent. Mix in garlic, thyme, nutmeg, salt, and pepper; cook for 1 more minute.

Stir in the heavy cream and simmer gently for 2 minutes.

In a greased baking dish, layer half of the sliced potatoes, pour half of the cream sauce over them.

Sprinkle half of the Gruyère cheese over the first layer, then add the remaining potatoes and cream sauce, topping with the rest of the cheese.

Bake for 1 hour and 10 minutes until golden brown and tender.

Allow to rest for about 10 minutes before serving.