Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the heavy cream, whole milk, minced garlic, salt, black pepper, and nutmeg. Whisk until well combined.
Grease a 9x13-inch baking dish with butter or cooking spray.
Layer half of the sliced potatoes in the bottom of the baking dish. Pour half of the cream mixture over the potatoes, followed by half of the Gruyère and cheddar cheeses.
Repeat the layering process with the remaining potatoes, cream mixture, and cheeses.
Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes.
Remove the foil and bake for an additional 30 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork.
Let the bake cool for about 10 minutes before serving. Garnish with chopped parsley.