Preparation

• Position a rack in the center of the oven, and heat the oven to 400°F. Grease a 3-qt. gratin dish with 1 Tbs. of the butter.
• Melt the remaining 2 Tbs. butter in a heavy-based medium saucepan over low heat. Add the flour, and stir well to form a paste. Cook gently while stirring, 1 to 2 minutes. Add the cardamom, cinnamon, ginger, and Aleppo pepper. Gradually add the broth a little at a time, whisking well between each addition to break up any lumps. Once all the broth has been incorporated, increase the heat to medium, and cook, stirring, until the sauce has thickened, 1 to 2 minutes. Season with 1/2 tsp. salt and 1/4 tsp. black pepper, and then add the nutmeg. Stir in the cream, cilantro, and orange zest. Remove from the heat and cover.
• In a medium skillet, heat 1 Tbs. of the oil over low heat, and cook the onion, stirring occasionally, until softened, 4 to 5 minutes. Add the garlic and cook until fragrant, about 30 seconds. Set aside.
• Peel the sweet potatoes. Using a sharp knife or mandoline, slice the sweet potatoes crosswise into 1/8-inch-thick rounds.
• Layer the sweet potato slices with the onion mixture in the prepared dish, seasoning each layer with 1/2 tsp. salt and 1/4 tsp. pepper and gently pressing down on the potatoes to ensure even layers. You should end up with about 8 layers.
• Uncover, and gradually bring the spiced sauce to a boil over low heat, stirring occasionally. Pour the sauce over the sweet potatoes, making sure that the potatoes are evenly covered and pushing on the slices gently with a wooden spoon to distribute the sauce.
• Sprinkle the nuts and panko over the potatoes in an even layer. Drizzle with the remaining 2 Tbs. oil. Place the gratin dish on a large rimmed baking sheet, and cover the dish with a sheet of oiled foil. Bake for 45 minutes.
• Uncover and bake for an additional 30 minutes until the top is golden-brown.
• Let the gratin rest for at least 20 minutes, and then slice and serve.