This tangy pie tastes best when it has been chilled overnight in the refrigerator.
Preheat oven to 350 degrees. Place crust (in its pan) on a baking sheet. Brush crust lightly with some egg white; discard remainder. Bake until crisp and slightly darkened, 6 to 8 minutes. Set aside to cool.
Make the Filling: In a food processor, process sugar and flour until well combined. Add eggs; process until smooth. Add buttermilk, butter, and salt; process just until blended. Pour into crust.
Make the Raspberry Sauce: Combine raspberries, sugar, and lemon zest in a bowl. Lightly mash about half the raspberries with a fork. Let sit at least 10 minutes to allow juices to release.
Bake until filling is almost set but center still jiggles slightly when pan is gently shaken, 35 to 40 minutes.
Transfer pie to a rack; cool completely. Cover, and refrigerate at least 2 hours and up to 2 days. To serve, slice pie and drizzle slices with raspberry sauce.