1. Preheat your oven to 375 F (190 C). Place the peeled and cubed sweet potatoes into a large pot and cover them with cold water. Bring the water to a boil over medium-high heat and cook until the sweet potatoes are fork-tender, which usually takes about 15-20 minutes. Drain them thoroughly in a colander.

2. Transfer the drained sweet potatoes to a large mixing bowl. Add 1/2 cup melted unsalted butter, granulated sugar, light brown sugar, milk, vanilla extract, ground cinnamon, ground nutmeg, and a pinch of salt. Using a potato masher or a large fork, mash the sweet potatoes until they are smooth and creamy. A few small lumps are perfectly fine and add a nice texture.

3. Lightly grease a 9×13-inch baking dish with butter or cooking spray. Pour the mashed sweet potato mixture into the prepared dish and spread it evenly with a spatula.

4. In a separate medium bowl, combine the all-purpose flour, 2 tablespoons of melted unsalted butter, and chopped pecans. Use your fingers or a fork to mix these ingredients until the mixture resembles coarse crumbs. Ensure the butter is well incorporated and coats the flour and pecans.

5. Evenly sprinkle the pecan crumble topping over the mashed sweet potatoes in the baking dish. Make sure to cover the entire surface, creating a beautiful, crunchy layer.

6. Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the topping is golden brown and crispy, and the sweet potato mixture is heated through and bubbly around the edges.

7. Remove the casserole from the oven and let it cool for a few minutes before serving. This allows the topping to set a bit and prevents any accidental burns from the hot filling.