Preheat the oven to 350F. Grease a 9x13 inch baking dish.
In a large bowl, mix together the mashed sweet potatoes, brown sugar, melted butter, eggs, whole milk, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the sweet potato mixture into the prepared baking dish.
In a separate bowl, mix together the chopped pecans, flour, brown sugar, and melted butter until crumbly. Sprinkle the pecan topping over the sweet potato mixture.
Bake for 45 minutes, or until the top is golden brown and the filling is set.
Let the sweet potato casserole cool for a few minutes before serving.
