Preheat oven to 400, butter a 9×13 baking dish and set aside
Melt 2 tablespoons of butter in a large pot over medium low heat, then add onions, stirring, until softened, about 5 minutes. Add kale, pinch of nutmeg, salt and pepper and cook, stirring, until kale has wilted, about 8 minutes. Remove the pot from the heat & set aside
Combine milk and garlic in small saucepan and bring to a simmer; keep warm. Melt two tablespoons butter in a medium saucepan over medium heat and stir in flour to make the roux. Whisk for one minute, then slowly whisk in warm milk and boil, whisking for one minute. Season with sea salt & pepper
Layer half of the sweet potatoes in the buttered baking dish. Then layer with half of the herbs and 1/3 of the cheese. Distribute half of the kale & onions mixture over the cheese, then pour half of the bechamel sauce on. Repeat this with the rest of the potatoes, the remaining herbs, 1/3 of the cheese, the rest of the kale & onions, then the remaining béchamel and finish with the last 1/3 of the cheese on top
Bake gratin for about 1 hour until golden and bubbly, and most of the liquid is absorbed. At an hour, my liquid wasn’t fully absorbed but the top was pretty brown. I turned the heat down to 240 and cooked for another 20 minutes. Let stand 10 minutes before serving