Preheat your oven to 350°F.

Grease a 10-inch cast-iron skillet generously with unsalted butter.

Peel and thinly slice the Yukon Gold potatoes to 1/8 to 1/4-inch thickness.

In a large bowl, combine the shredded Gruyère, grated Parmesan, minced garlic, picked thyme leaves, heavy cream, salt, pepper, and nutmeg. Stir well.

Spoon half of the creamy mixture into the prepared skillet, spreading it out evenly.

Place the potato slices in a spiral pattern, starting from the outer edge and moving inward.

Pour the remaining cream mixture over the arranged potatoes.

Place the skillet on a rimmed baking sheet and bake for 1 hour 40 minutes to 1 hour 50 minutes.

Allow the dish to cool for 10 minutes before serving.