1. Prepare the Sweet Potatoes: Begin by peeling your sweet potatoes and cutting them into roughly 1-inch cubes. Place the cubed sweet potatoes in a large pot and cover them with cold water. Bring the water to a boil over medium-high heat, then reduce the heat and simmer until the sweet potatoes are fork-tender, about 15-20 minutes. You should be able to easily pierce them with a fork.

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 the 1/2 cup of melted butter, granulated sugar, 1/4 cup light brown sugar, milk, vanilla extract, 1/2 teaspoon ground cinnamon, nutmeg, and salt. Mash the sweet potatoes with a potato masher or an electric mixer until smooth and creamy. Taste and adjust seasoning if desired.

3. Assemble the Casserole Base: Pour the mashed sweet potato mixture into a greased 9×13-inch baking dish, spreading it evenly with a spatula.

4. Prepare the Pecan Crumble Topping: In a separate medium bowl, combine the all-purpose flour, 1/2 cup light brown sugar, 1 cup chopped pecans, and 1/4 teaspoon ground cinnamon. Cut the 4 tablespoons of cold butter into small pieces and add them to the flour mixture. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

5. Add the Topping: Sprinkle the pecan crumble mixture evenly over the sweet potato base in the baking dish. Ensure a good, consistent layer for that perfect crunch in every bite.

6. Bake the Casserole: Preheat your oven to 375 F (190 C). Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the topping is golden brown and the sweet potato mixture is heated through and bubbly around the edges.

7. Rest and Serve: Once baked, remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set slightly and makes for easier scooping.