Add flour, sugar, baking powder, and salt to a food processor and pulse three times.
Add cold butter and pulse until crumbly.
Add egg and pulse until dough forms a ball.
Wrap and chill for 15–20 minutes.
Roll out dough, fit into a pie pan, and refrigerate.
Preheat oven to 375°F. Brush crust with beaten egg white.
Par-bake crust for 7–8 minutes. Let cool 10 minutes.
Whisk all custard ingredients in a large bowl until smooth.
Pour mixture into crust. Sprinkle with nutmeg.
Bake 35–45 minutes or until a knife comes out clean.
Cool completely, then chill in fridge 4+ hours before serving.
