1. Roast the Sweet Potatoes:
Preheat your oven to 400°F (about 200°C). Line a baking sheet with parchment paper. In a large bowl, toss the cubed sweet potatoes with the olive oil, ground cinnamon, and nutmeg until they’re fully coated. Arrange the sweet potatoes in a single, even layer on the baking sheet. Bake for 50–60 minutes, or until the sweet potatoes are tender, golden and slightly charred. When roasting is complete, take them out of the oven and allow them to cool for a few minutes.

2. Make the Casserole Filling:
Lower the oven temperature to 375°F (about 190°C) and lightly grease a large 9×13‑inch baking dish. In a bowl, combine the roasted sweet potatoes, milk, vanilla extract, brown sugar, maple syrup, the second measure of cinnamon, nutmeg, allspice, raisins (soaked and drained) and marshmallows. Mix until the ingredients are evenly incorporated — it’s okay if the sweet potato pieces start to break down and you end up with a mixture of chunks and mashed bits. Scoop the mixture into the prepared baking dish and spread it out evenly. Distribute the small pieces of butter by tucking them into different parts of the casserole mixture. For extra gooey goodness, you can press in a few additional marshmallows into the sweet‑potato mix.

3. Make the Topping & Bake:
In a separate bowl, combine the raw pecans with the maple syrup, stirring until the pecans are coated. Generously sprinkle this topping over the sweet potato mixture so that it’s fully covered. Cover the baking dish with foil and bake for about 15 minutes. Then remove the foil and continue baking for another 15 minutes (or until the topping is golden and the marshmallows have melted). Once finished, remove the casserole from the oven and let it rest for a few minutes before serving.