Partially bake the Pie Crust: Preheat the oven to 350°F, roll out the pie dough and line a 9 inch pie pan. Chill at least 1 hour. Place a piece of crumpled parchment paper in side and fill with pie weights or beans. Bake in preheated oven for 15-20 minutes or until the sides begin to turn a golden brown. Remove the parchment with the weights. You can set it aside to cool or use it warm.
Warm the milk in a sauce pot over low heat on the stovetop or in the microwave. Keep warm.
Scrub and wash the sweet potato skins well. Prick a few times with a fork for roasting or peel and cut into large pieces for boiling.
You can boil the peeled potatoes or roast them (350°F) in a baking dish covered with foil until soft and easily pierced with a fork, cake tester, or knife.
Wait until just cool enough to handle and remove the skins. You can simply squeeze it out or scoop it with a spoon. Place the hot sweet potato flesh in a stand mixer bowl fitted with the paddle attachment or in a large mixing bowl.
Beat the butter and sweet potatoes in the bowl of a stand mixer on medium speed or with an electric mixer. The hot potatoes will melt the butter and make a smooth mixture.
Gradually add sugar, beating well after each addition until smooth.
Add the eggs, one at a time; beat after each addition and scrape down the bowl.
Add the warm milk, lemon zest, lemon juice, vanilla, and nutmeg to the sweet potato mixture; stir until fully combined. You can also use a whisk.
Pour the mixture into the partially baked pastry shell. The mixture will still be a little warm, which is ideal.
Bake the pie at 350°F until a knife inserted into the center comes out clean, about 35-40 minutes.
Cool to room temperature before refrigerating. Chill for 2 hours or overnight before slicing and serving. The pie is delicious at room temperature or cold.