!For the Crust:
Preheat oven to 350
Mix the flour and salt in a bowl. Add the vegetable shortening and salted butter. Work the butter into the flour using a dough cutter until the mixture resembles crumbs.
Add the egg, 5 tablespoons cold water and the white vinegar. Stir until just combined. Divide the dough in half (you will only need one half for this recipe, reserve the other half for another use).
Roll the dough out to fit a 9-inch deep dish pie plate. Ease the dough into the pie plate and crimp edges as desired. Pierce the bottom and sides of dough with a fork. Bake 20-30 minutes, until it begins to just turn golden in spots. Allow to cool.
!For the filling:
In a stand mixer fitted with the whisk attachment, beat butter and sugar together until combined, about 1 minute. Add the yolks and beat until pale and thick, another minute or so.
Reduce the mixer speed and add the flour, lemon zest and juice, nutmeg, and salt until just incorporated.
Increase speed to medium and gradually pour in the buttermilk. Scrape sides and bottom of bowl, beating again until just completely combined. Transfer mixture to a large bowl.
Clean mixing bowl and whisk. Add the egg whites and beat on medium-high speed until stiff peaks form, about 4 minutes. Gently fold the eggs whites into the buttermilk mixture.
Pour the filling into the prepared pie crust. Place pie plate on a baking sheet and bake 45-50 minutes until golden and set (the center of pie will still jiggle slightly).
Let cool on rack to room temperature. Dust with powdered sugar, if using, before serving. Store in refrigerator (also good cold).