Steam kale for about 3 minutes, or until wilted, in a large steamer basket set over boiling water. Remove from heat and allow to cool slightly. Squeeze out excess water.
Place half of the potatoes in a greased 9 x 13 baking dish. (You don't have to do this neatly, but try to make sure each layer is even!) Top potatoes with kale, then top kale with half of the Parmesan cheese. Place remaining potatoes in dish and sprinkle with the rest of the cheese. Whisk together whipping cream, salt, pepper, and nutmeg and pour over potatoes. Dot with butter.
Cover baking dish with aluminum foil and bake for 25 minutes. Remove foil and bake for 20-25 minutes more, or until potatoes are tender and cheese is golden brown. Allow to sit 15 minutes before serving.
Notes
To make this more of an everyday side dish, you can use whole milk in place of the heavy whipping cream, but this increases the likelihood of the gratin curdling. If the gratin curdles, don't panic! It is still perfectly fine to eat, it just doesn't look very pretty.
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