Put the peeled sweet potato chunks in a medium saucepan and cover them with water. Bring the pot to a boil, then lower the heat, cover, and simmer for 20 to 30 minutes. They are ready when a fork slides in with no resistance.
Drain the potatoes well in a colander so no extra water gets into your filling. While they sit, heat your oven to 350°F. Set the frozen crust on a baking sheet if you like, so it is easy to move.
Tip the warm potatoes into a large bowl and mash them thoroughly. Take out as many lumps as you can now, because they will not disappear later. A smooth base means a silky pie.
Add the evaporated milk, melted butter, brown sugar, beaten eggs, cinnamon, vanilla, and nutmeg to the bowl. Stir until everything looks even and creamy. The mixture should be thick but pourable.
Pour the filling into the unbaked deep-dish crust. Smooth the top with a spoon so it bakes evenly. Fill it carefully, since the filling will be generous.
Bake the pie for 40 to 45 minutes. Insert a knife near the center. If it comes out clean, the custard is set. If not, give it a few more minutes.
Move the pie to a wire rack and let it cool fully. This step lets the filling firm up so each slice holds its shape. Be patient, because it is worth the wait.