1. Prepare the Sweet Potatoes: Begin by placing the peeled and cubed sweet potatoes into a large pot. Cover them with cold water and bring to a boil over medium-high heat. Cook until the sweet potatoes are very tender when pierced with a fork, which usually takes about 15-20 minutes. Once tender, drain them thoroughly in a colander.

2. Mash the Sweet Potato Base: Transfer the drained sweet potatoes to a large mixing bowl. Add the softened butter, granulated sugar, milk, vanilla extract, ground cinnamon, ground nutmeg, and salt. Use a potato masher or an electric mixer to mash the sweet potatoes until they are smooth and creamy. Make sure there are no large lumps remaining.

3. Prepare for Baking: Lightly grease a 9×13-inch baking dish with butter or cooking spray. Spread the mashed sweet potato mixture evenly into the prepared baking dish. Use the back of a spoon or a spatula to smooth the top.

4. Create the Pecan Topping: In a separate medium-sized bowl, combine the all-purpose flour, packed light brown sugar, and chopped pecans. Pour the melted butter over these dry ingredients. Use a fork to mix everything together until the mixture is crumbly and well combined. The texture should resemble coarse crumbs.

5. Add the Topping: Evenly sprinkle the pecan topping mixture over the sweet potato base in the baking dish. Make sure to cover the entire surface for a consistent crunch in every bite.

6. Bake to Golden Perfection: Preheat your oven to 375 F (190 C). Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the sweet potato mixture is heated through and the pecan topping is golden brown and slightly crisp. Keep an eye on the topping to prevent it from burning.

7. Serve Warm: Once baked, remove the casserole from the oven and let it cool for a few minutes before serving. This allows the flavors to meld and the casserole to set slightly.