Step 1: Roast the Sweet Potatoes
Preheat your oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper for easy cleanup. In a large bowl toss the sweet potato cubes with olive oil, cinnamon, and nutmeg until each cube is well coated. Spread them out in a single layer on the baking sheet so they roast evenly. Bake for 50 to 60 minutes, or until the potatoes are tender and begin to char and caramelize on edges. Once done, remove from the oven and let them cool slightly so they are easier to handle.

Step 2: Prepare the Casserole Filling
Lower the oven temperature to 375 degrees Fahrenheit. Lightly grease a large 9 by 13 inch baking dish. In a big bowl combine the roasted sweet potatoes with almond milk, vanilla extract, brown sugar, maple syrup, cinnamon, nutmeg, allspice, raisins, and marshmallows. Stir gently until everything is blended. Some potatoes will start to mash and others will stay in chunks. That unevenness gives a lovely texture contrast. Transfer this mixture into the prepared baking dish and spread evenly. Tuck the small cubes of butter throughout the mixture. If you like extra marshmallow pockets, gently press a few more marshmallows into the top of the mixture.

Step 3: Add the Maple Pecan Topping
In a small bowl mix the raw pecans with maple syrup until the nuts are coated evenly. Sprinkle this mixture generously over the sweet potato filling in the baking dish so the top is fully covered with pecans and syrup.

Step 4: Bake the Casserole
Cover the dish with foil and bake in the preheated oven for 15 minutes. Then remove the foil and bake for another 15 minutes, or until the topping is golden brown and the marshmallows have melted nicely. After baking, remove the casserole from the oven and allow it to rest for a few minutes before serving. This short cooling period helps the filling firm up, making it easier to slice and serve.