Preheat your oven to 375°F (190°C). Butter a 9x13-inch baking dish well so the gratin slides out easily after baking.

Using a mandoline or a very sharp knife, slice your potatoes about 1/8-inch thick. As you slice, place the potatoes in a bowl of cold water to keep them from browning.

In a medium saucepan over low heat, warm the heavy cream with the minced garlic, salt, pepper, and nutmeg. Heat until fragrant and just starting to steam — don’t let it boil. Remove from heat and stir in 1 cup of the Gruyère and 1/2 cup Parmesan until melted and smooth.

Drain the sliced potatoes and pat dry with a clean towel. Arrange a single overlapping layer of potato slices in the prepared dish. Spoon about a quarter of the cheesy cream over that layer, spreading gently. Repeat until all the potatoes and cream are used, finishing with a thin layer of cream.

Sprinkle the remaining 1/2 cup Gruyère and 1/2 cup Parmesan evenly over the top. If desired, mix the panko breadcrumbs with a tablespoon of melted butter and scatter them across the cheese layer.

Cover the dish tightly with foil and bake for 40 minutes. Remove the foil and bake for another 20–25 minutes, until the top is bubbling and golden brown.

Let the gratin rest for 10–15 minutes before serving. Garnish with chopped chives or parsley and serve warm.