Preheat oven to 180 C (350 F). Place sweet potatoes in a large saucepan and cover with cold water.
Bring to a boil, then simmer 15-20 minutes until fork-tender. Drain well - steam will escape and sweet, earthy aromas will rise.
Return potatoes to the pan and mash until smooth, or use a hand mixer for a silkier finish. The texture should be creamy, not gluey.
Stir in melted butter, brown sugar, eggs, milk, vanilla, cinnamon, and nutmeg. Taste and adjust sugar or spice as needed.
Spoon the sweet potato mixture into a greased 9x13-inch baking dish, smoothing the top with a spatula so it bakes evenly and sets uniformly.
For the topping, combine chopped pecans, flour, granulated sugar, and remaining butter until crumbly. Rub between fingers to form coarse clumps.
Scatter the pecan topping evenly over the sweet potatoes, pressing lightly so it adheres. You should see golden pockets of butter and sugar.
Bake for 15-20 minutes until the topping is golden and crisp and the filling is warmed through. The kitchen will smell of toasted nuts and brown sugar.
Let rest 10 minutes before serving so the layers set. The contrast of creamy interior and crisp topping should be pronounced.