1. Prepare the Sweet Potatoes: Begin by placing the peeled and cubed sweet potatoes in a large pot. Cover them with cold water and bring to a boil over medium-high heat. Reduce the heat to medium and simmer for 15-20 minutes, or until the sweet potatoes are fork-tender. This ensures a smooth, creamy base for your casserole.

2. Mash the Sweet Potatoes: Once tender, drain the sweet potatoes thoroughly. Return them to the pot or transfer them to a large mixing bowl. Add 1/2 cup of melted butter, brown sugar, milk, vanilla extract, cinnamon, nutmeg, and a pinch of salt. Mash with a potato masher or an electric mixer until smooth and creamy. Taste and adjust seasonings if needed.

3. Transfer to Baking Dish: Spoon the mashed sweet potato mixture evenly into a 9×13-inch baking dish. Use the back of a spoon or a spatula to smooth out the top. This creates a nice, even layer for your topping.

4. Prepare the Pecan Topping: In a medium bowl, combine the chopped pecans, all-purpose flour, and the remaining 2 tablespoons of melted butter. Stir well until the pecans are evenly coated and the mixture is crumbly. This step is crucial for achieving that signature crunchy texture.

5. Add the Topping: Sprinkle the pecan topping evenly over the mashed sweet potato layer in the baking dish. Make sure to distribute it across the entire surface so every serving gets some of that delicious crunch.

6. Bake the Casserole: Place the baking dish in a preheated oven at 375 F (190 C). Bake for 20-25 minutes, or until the sweet potato mixture is heated through and the pecan topping is golden brown and fragrant. The edges of the topping should look slightly toasted.

7. Serve Warm: Carefully remove the casserole from the oven. Let it cool for a few minutes before serving. This allows the flavors to meld and the topping to set a bit, making it easier to serve.