Position a rack in the center of the oven, and preheat the oven to 325° F.
If the crust is cool, warm it in the oven while the oven preheats for about 5 minutes. Keep an eye on it, it may start to puff up, and you don’t want it to puff too much, so remove it from the oven if you see it puffing.
Brush the crust all over with the whisked egg yolk.
Make the filling:
Place the whole eggs, 3 egg yolks, sugar, vanilla, and salt, in a medium mixing bowl, and whisk to combine.
Place the milk in a small pan, and bring it to a simmer over medium heat.
Gently whisking all the while, gradually add the hot milk to the egg mixture, and immediately pour the hot custard into the warmed pie crust.
Sprinkle the top of the custard with nutmeg, if desired.
Carefully place the pie on a cookie sheet, and bake in the center of the oven until the custard seems set but still but still jiggles a bit, like gelatin, about 35 to 45 minutes.
Remove from oven and let cool completely on a cooling rack, then refrigerate. Pie can be served cold from the fridge, or at room temperature.