Step 1: Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish with the butter.
Step 2: In a small saucepan, combine the heavy cream, minced garlic, thyme, nutmeg, salt, and pepper. Heat over medium heat until it just begins to simmer, then remove from heat.
Step 3: Arrange a single layer of sliced sweet potatoes in the bottom of the prepared dish, overlapping them slightly.
Step 4: Top the potatoes with a handful of chopped kale and a light sprinkle of Gruyère cheese.
Step 5: Repeat the layers of sweet potatoes, kale, and cheese until all ingredients are used, ending with a layer of sweet potatoes on top.
Step 6: Pour the warm cream mixture evenly over the entire dish. Cover the dish tightly with aluminum foil.
Step 7: Bake for 30 minutes. Remove the foil, sprinkle the remaining Gruyère cheese over the top, and bake for another 20-25 minutes until the potatoes are tender and the top is bubbly and golden brown.
Step 8: Let the gratin rest for 10 minutes before serving to allow the sauce to set.