Bake the sweet potatoes. Preheat the oven to 425°F/218°C. Lightly slash the sweet potatoes several times with a sharp knife to allow for even ventilation. Place the potatoes on a baking sheet (I line mine with parchment paper for easy cleanup). Bake for 45-55 minutes, or until the potatoes are fork tender.
Meanwhile, prepare the crumble topping. In a medium bowl, mix together the oats, walnuts, pecans, almond meal, brown sugar or coconut sugar, salt, cinnamon, and nutmeg. Then pour in the melted vegan butter and maple syrup and fold in with a fork until crumbly and just slightly sticky.
Once the baked sweet potatoes are cool enough to handle, slip off the peels and discard. Transfer the potatoes a large bowl.
Reduce the oven temp to 375°F/190°C. Lightly grease a 2-quart / 2 liter baking pan.
Mash the potatoes. Use a potato masher, ricer, or handheld electric mixer to break up the sweet potatoes and mash them but not until completely smooth.
Brown the vegan butter. Heat a light-colored skillet over medium heat, then add the butter. Once the butter is melted, stir often and swirl the pan to ensure even browning and cooking, until it foams and then eventually browns and smells nutty, about 5-7 minutes.
Make the mashed sweet potatoes. Take the browned butter off the heat and immediately pour into the mashed sweet potatoes (if your potatoes aren’t ready yet, pour it into a bowl to prevent the butter from burning). Use a silicone spatula or wooden spoon to fold the browned butter into the mashed potatoes. Add in the vanilla extract, cinnamon, ginger, nutmeg, salt and black pepper to taste. Fold until all ingredients are incorporated and potatoes are creamy and smooth.
Assemble the casserole. Add the mashed sweet potatoes to the prepared baking dish and smooth out the surface. Sprinkle the crumble topping over the mashed sweet potatoes.
Bake the casserole in the preheated oven for 35-40 minutes, or until the sweet potato mixture just starts to pull away from the edges of the pan and the topping is nicely golden brown. Let the casserole cool for 10-20 minutes before serving.