1. Prepare the Sweet Potatoes: Begin by peeling the sweet potatoes and cutting them into 1-inch chunks. Place them in a large pot and cover with cold water. Bring to a boil over medium-high heat and cook until they are very tender when pierced with a fork, usually about 15-20 minutes. Drain them thoroughly.

2. Mash the Sweet Potatoes: Transfer the drained sweet potatoes to a large mixing bowl. Add 1/4 cup of the melted butter, 1/4 cup packed light brown sugar, granulated sugar, milk, vanilla extract, cinnamon, nutmeg, and salt. Mash with a potato masher until smooth and creamy. A hand mixer can also be used for an extra smooth consistency.

3. Assemble the Casserole Base: Pour the mashed sweet potato mixture into a 9×13-inch baking dish, spreading it evenly with a spatula. You want a nice, flat layer for the topping to rest on.

4. Prepare the Pecan Topping: In a separate medium bowl, combine the chopped pecans, all-purpose flour, 1/4 cup packed light brown sugar, and the softened butter. Use your fingers or a pastry blender to mix these ingredients until crumbly and well combined. The mixture should resemble coarse crumbs.

5. Add the Topping and Bake: Sprinkle the pecan crumble topping evenly over the sweet potato mixture in the baking dish. Make sure it covers the entire surface.

6. Bake to Golden Perfection: Preheat your oven to 375 F (190 C). Place the casserole in the preheated oven and bake for 30-35 minutes, or until the sweet potato mixture is heated through and the pecan topping is golden brown and slightly crispy.

7. Serve Warm: Remove the casserole from the oven and let it cool for a few minutes before serving. The topping will crisp up a bit more as it cools slightly.