Blend dry ingredients and butter:Mix flour and sea salt in a large mixing bowl. Add the butter and blend using pastry blenders or your fingers until you have a bowl filled with crumbs.
Make pastry:Mix vinegar and water in a measuring jug. Slowly pour over dry ingredients and mix just until it comes together into a dough. Push into a ball. Do not knead or overwork! Divide into 2 disks, wrap in foil and chill for 1 hour.
Roll out bottom crust: Place 1 disk of the pastry between 2 sheets of baking parchment. Roll into a circle slightly larger than a 9 inch pie dish. Fit into the lightly greased pie dish and place back into the fridge to chill.
Prep:Preheat oven to 420°F.
Mix filling ingredients:Place all ingredients for the filling in a large bowl and mix well. Pour into prepared bottom crust. Evenly dot with the diced butter.
Add top crust:Remove the second disk of dough from the fridge and roll into a 10 inch circle between two sheets of baking parchment. You can either place it on top as a whole (make sure to cut some air vents into the top if doing this!) OR cut it into strips to make a lattice. The lattice is a bit of a pain with this crust... But it looks pretty and helps the filling to thicken and set as more liquid can evaporate.
Crimp edge:Pinch together the edges of the top and bottom crust, then crimp. Brush with egg white and sprinkle with granulated sugar.
Bake the pie:
Hot bake:Bake the pie on the bottom rack for 10 minutes at 420°F. Place a pie shield or aluminum foil over the edge once these 15 minutes are up.
Finish Baking:Lower the heat to 375°F and finish baking the pie for 35-45 minutes, or until the filling is bubbly and the crust is baked. Cool on a wire rack before slicing.
To get neat slices, cool overnight. If you want to eat the pie warm after a shorter cooling period, the filling will be a little more lose like in a crisp. But it's delicious either way!
If using frozen fruit, add 2 more tablespoons of cornstarch to filling.
Additional white sugar is fine to use in place of the brown sugar.
If rhubarb stalks are very coarse/thick, peel before slicing.
Store baked and cooled pie covered with plastic wrap at room temperature for up to 2 days.