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

2. Return the drained sweet potatoes to the empty pot or transfer them to a large mixing bowl. Add 1/4 cup of the melted butter, brown sugar, milk, vanilla extract, cinnamon, nutmeg, and a pinch of salt. Mash with a potato masher until smooth and creamy. If you prefer a very smooth texture, you can use an electric mixer.

3. Spread the mashed sweet potato mixture evenly into a 9×13-inch baking dish. Make sure it’s distributed from edge to edge for even baking.

4. In a separate medium bowl, combine the all-purpose flour, chopped pecans, and granulated sugar. Pour the remaining 1/4 cup of melted butter over the dry ingredients.

5. Use a fork or your fingertips to mix the butter into the flour, pecans, and sugar until the mixture resembles coarse crumbs. Avoid overmixing; you want distinct crumbles, not a paste.

6. Sprinkle the pecan crumble mixture evenly over the mashed sweet potatoes in the baking dish.

7. Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown and the sweet potato mixture is heated through and bubbly around the edges.

8. Let the casserole cool for a few minutes before serving. This allows the topping to set slightly and prevents burns.