To a large bowl, mix together 150 grams flour with sugar and salt.

Cut the cold, unsalted butter into small chunks and add to the flour mixture. Using a pastry cutter, cut the butter into the flour mixture until all of the butter is moistened by the flour and forms a thick paste.

Add the remaining 175 grams of flour and use the pastry cutter to distribute the flour until the mixture looks crumbly.

Pour the chilled sourdough discard on top of the flour/butter mixture and stir to combine. Use your hands to form the dough into a ball. If the dough is too crumbly, add ice cold water a Tablespoon at time until it comes together.

Cut the ball in half and form two balls of dough. Wrap the balls in plastic wrap, press down on them to form a disc shape and stick in the refrigerator to chill.

While the dough is chilling, make the pie filling. Chop the rhubarb into small 1 inch pieces. Add them to a large bowl. Pour sugar and flour over the rhubarb and stir together. Set aside.

Preheat the oven to 400ºF.

Roll out the first disc of dough a little over 10 inches round. Fold the dough into quarters and place it inside a pie plate, unfolding it and fitting it to the pie plate.

Fill the bottom pie crust with the rhubarb pie filling. Cut up 2 Tablespoons of butter and dot all over the pie. Place the second pie crust on top of the pie filling.

Crimp the edges of the pie and add two or more slits in the pie crust to allow steam to escape while baking. Brush the top with egg wash and sprinkle with sugar.

Bake the pie for 30 minutes. Then cover with foil and bake for another 30 minutes until the juices are bubbling.

Let pie cool completely before serving.