Preheat the oven to 375F and arrange the rack in the center. Butter a 3 QT Carbon Steel Gratin or 10 x 6.6” Oval Gratin Baking Dish. Separate the green leaves from the stems, stack the leaves, and slice into ½-inch ribbons. Discard the stems, or thinly slice them into ¼-inch pieces if using.

Heat a large Carbon Steel or Stainless Clad Frying Pan over medium. Melt 2 tablespoons butter, add half the garlic, and cook, stirring, until fragrant, about 30 seconds. Add the stems, if using, and cook until softened, about 2 minutes. Add the leaves, season with salt and pepper, and cook until wilted and most of the liquid has evaporated, 3–5 minutes. Transfer to a Sheet Pan or Entrée Bowl and let cool slightly.

In a large bowl, combine 1 ½ cups cream, ½ cup Gruyère, ½ cup Parmesan, remaining garlic, cayenne, and nutmeg. Thinly slice the potatoes, about ⅛-inch thick, using a mandoline (preferred) or sharp knife (no need to peel the potatoes). Quickly add the slices to the cream mixture to prevent oxidation. Season with ¾ teaspoon salt and freshly ground black pepper and mix well to coat.

Shingle slightly overlapping slices of potato into the prepared dish to form a solid layer, tucking smaller pieces into gaps, until you’ve used about half the potatoes. Top with half of the cream mixture and another ½ cup each Gruyère and Parmesan.

Spread the cooked greens evenly over the potatoes, leaving behind any excess liquid. Layer the remaining potatoes on top with the remaining cream mixture, pour the remaining 1 cup cream over (but do not completely submerge), and sprinkle with the rest of the Gruyère and Parmesan.

Cover with foil and bake until potatoes are very tender when tested with a Paring Knife, 60–90 minutes. Remove the foil and bake until the top is deeply golden brown, another 30 minutes more. Let rest 10 minutes before serving. Garnish with chives or parsley.