Step 1

Preheat the oven to 375 degrees F.

Step 2

For the sweet potatoes: Put the sweet potatoes in a large pot and fill with enough water to cover by 2 inches. Season with a good pinch of salt and bring to a boil over medium-high heat, lower to medium low and simmer until the potatoes are tender, 8 to 10 minutes. Drain well and return the potatoes to the pot.

Step 3

Add the coconut milk, brown sugar, vanilla, cinnamon, nutmeg and a pinch of salt; use a potato masher to mash the sweet potatoes until the mixture very smooth (see Cook's Note). Taste and adjust the seasoning with more salt if needed. Transfer to a 2-quart baking dish and smooth into an even layer.

Step 4

For the nut topping: While the potatoes are cooking, add the walnuts, pecans, flour, brown sugar, cinnamon and a pinch of salt to a food processor and pulse a few times until evenly and finely chopped (the nuts should be small but not pulverized). Add the coconut oil and pulse a few more times until the mixture is fully moistened and combined.

Step 5

Sprinkle the nut mixture evenly over the sweet potato filling. Bake until the top is golden brown and the filling is heated through, 25 to 30 minutes. Let the casserole cool 5 minutes before serving.