Preheat the oven to 400°F with a rack in the center. In a small saucepan, add the cream, garlic, nutmeg, and 3 tablespoons of the ghee and set over low heat, fill a large bowl with cold water, peel and slice the potatoes into ⅛-inch-thick slices and immediately add the potatoes to the bowl, grease a baking dish with the remaining tablespoon of ghee.
Drain the potatoes and pat them completely dry with paper towels, set aside, arrange a single layer of potatoes on the bottom of the baking dish, lightly season with salt and pepper and sprinkle with ⅓ cup gruyere cheese, repeat with the remaining potatoes. Then, slowly pour the warm cream mixture over the potatoes, then gently press down with a spatula to make sure the liquid is evenly distributed, sprinkle it with remaining gruyere and Parmesan cheese.
Cover the dish with foil and bake covered for 50 minutes, then remove foil and continue to bake for another 25 minutes until bubbling, garnish with parsley, if using.
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