Peel and thinly slice the Yukon Gold potatoes into ⅛-inch rounds.

In a medium saucepan over medium heat, melt the butter and sauté the minced garlic until fragrant, about 1 minute.

Whisk the heavy cream and whole milk into the pan. Season with salt, black pepper, and ground nutmeg, and heat gently to a simmer.

Stir in ¾ cup of the Gruyère cheese into the simmering cream mixture until fully melted.

Preheat the oven to 375°F (190°C) and grease a 9×13-inch baking dish.

Arrange half of the sliced potatoes in the greased baking dish, then pour over half of the creamy sauce.

Layer with remaining potatoes and pour the rest of the cream mixture over.

Sprinkle remaining Gruyère cheese and all of the Parmesan cheese on top.

Cover with aluminum foil and bake for 40 minutes. Uncover and bake for another 20-25 minutes until golden brown.

Allow to rest for about 10 minutes before serving and garnish with fresh parsley if desired.