Preheat oven to 375°F (190°C). Generously grease a 10-inch cast-iron skillet or 2-quart baking dish with 1 tablespoon of butter.

In a small saucepan or microwave-safe bowl, combine heavy cream, minced garlic, salt, pepper, and nutmeg. Heat gently until just warm.

Arrange one-third of the potato slices in an overlapping pattern in the skillet. Sprinkle with one-third of the Gruyère cheese. Pour one-third of the warm cream mixture over the top.

Repeat the layers twice more, ending with potatoes, then cream, then the final remaining cheese.

Dot the top with remaining 1 tablespoon of butter and sprinkle with fresh thyme. Cover tightly with aluminum foil.

Bake covered for 45 minutes.

Remove foil and bake uncovered for another 30 to 45 minutes, until the potatoes are very tender when pierced with a knife and the top is deeply golden brown and bubbly.

Let rest for at least 15-20 minutes before serving to allow the sauce to set.