1. Preheat and prepare the dish.
Preheat your oven to 350 °F (about 175 °C) and grease a 9 × 13 inch baking dish.

2. Cook the sweet potatoes.
Add the peeled and cubed sweet potatoes to a large pot. Pour in enough water to cover the cubes by about 1 inch. Bring the water to a boil, then reduce to a simmer and cook for about 15 minutes, or until the sweet potatoes are fork-tender. Drain thoroughly.

3. Mash the filling.
In a large bowl, combine the drained sweet potatoes with the brown sugar, melted butter, vanilla extract, cinnamon, salt and nutmeg. Mash everything together until smooth (or to your preferred texture). Evenly spoon the mashed mixture into the greased baking dish.

4. Make the pecan topping.
In a medium bowl, whisk together the melted butter, brown sugar and flour until combined. Stir in the chopped pecans. Distribute the pecan topping evenly across the sweet potato base.

5. Bake and serve.
Place the dish into the preheated oven and bake for about 25–30 minutes, or until the casserole is hot and bubbling and the pecan topping is toasted and golden. Serve warm and enjoy!