Preheat the oven to 375°F (190°C). Scrub the sweet potatoes, pierce them a few times with a fork, and place on a baking sheet. Bake until very tender, about 45–60 minutes depending on size. Alternatively, steam or boil until tender if you prefer faster cooking.

Once the sweet potatoes are cool enough to handle, peel them and transfer the flesh to a large bowl. Mash with a potato masher or pass through a ricer for an extra-smooth texture.

Add the butter, brown sugar, milk, vanilla, cinnamon, nutmeg, and salt to the mashed sweet potatoes. Mix until smooth and taste, adjusting seasoning or sweetness if needed.

Spoon the sweet potato mixture into a 9x13-inch (or similar) baking dish, spreading it into an even layer. Smooth the top with a spatula.

Make the oat streusel: In a medium bowl, combine rolled oats, flour, packed brown sugar, and salt. Add the cold butter cut into small pieces. Use a pastry cutter or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse crumbs. Stir in the chopped pecans if using.

Sprinkle about two-thirds of the oat streusel evenly over the sweet potato layer. This helps the potatoes heat through with a crunchy interior under the marshmallows.

Bake the casserole uncovered at 375°F (190°C) for 20 minutes to warm the filling and set the streusel. Remove from the oven and evenly distribute the mini marshmallows across the top, then sprinkle the remaining streusel over the marshmallows (this keeps the marshmallows from burning too quickly).

Return the casserole to the oven and bake for an additional 8–12 minutes, or until the marshmallows are puffed and golden and the streusel is crisp. For extra toasting on the marshmallows, place under the broiler for 30–60 seconds — watch very closely to avoid burning.

Let the casserole rest for 5–10 minutes before serving so the filling firms slightly. Serve warm and enjoy!