Slice the sweet potatoes into 1-inch rounds and toss with a little olive oil. Roast on a baking sheet in a preheated oven at 200°C (400°F) for about 40 minutes, until edges are caramelized and flesh is tender when pierced with a fork.
While the sweet potatoes roast, toast the pecans in a dry skillet over medium heat until fragrant and crunchy, about 3-4 minutes. Chop them coarsely and set aside.
Once the sweet potatoes are cool enough to handle, peel off the skins and transfer the flesh to a large mixing bowl. Mash with a potato masher or fork until smooth but still slightly chunky for texture.
Stir in the melted butter, vanilla extract, cinnamon, and maple syrup until well combined. The mash should be creamy and fragrant, with a hint of spice and sweetness.
Transfer the sweet potato mixture into a buttered 9x13-inch baking dish and spread evenly with a spatula.
Mix the toasted pecans with brown sugar and sprinkle evenly over the top of the casserole, pressing lightly to help them stick.
Bake the casserole at 180°C (350°F) for 20-25 minutes, until the topping is golden and bubbling. If using marshmallows, sprinkle them over the top during the last 5 minutes of baking.
Optionally, broil for 2-3 minutes to crisp the marshmallows or topping, watching carefully so it doesn't burn.
Remove from the oven and let rest for 5 minutes. The casserole will be hot and fragrant, with a beautifully caramelized topping.
Serve warm, garnished with extra toasted pecans or a drizzle of honey if desired, and enjoy this comforting, adaptable dish.
