Preheat your oven to 425°F with the baking rack in the lowest position in the oven.
Gently toss together your Saskatoons, sugar and cornstarch/crushed Tapioca, until the berries are coated.
Place your bottom crust into the pie plate, letting it drape into the bottom instead of pressing it in.
Pour your berries into the bottom crust. Add the cold pieces of butter on top.
Cover with your crust. Pinch the edges of the pie pastry together and slash 2-3 pie vents in the top of the pastry. This allows steam to release while it bakes. You can also make a lattice pie crust if wanted.
Bet the egg and the water together until combined. Brush your pie crust with the egg wash, then sprinkle the sugar on top in an even layer.
Bake the pie for 15 minutes at 425°F, then lower the temperature to 350°F and bake for about 50-55 minutes more. If you have an oven that runs "hot". then this should be 325°F, and it will take longer to bake.
When your pie edges start to become too brown - and they always will- cover the edges with tinfoil or use a pie crust cover.
Continue baking until you see the pie filling bubbling through the vents and your pie is beautifully browned all over the top. Again, if your pie crust is browning too fast, turn the oven down to 325°F.
Remove from the oven and cool on a baking rack completely. This pie will need to set for a few hours before slicing. You can refrigerate it once it is room temperature.
Slice and serve.
Notes
Depending on how juice the berries are, you might have to add another 1/2 tablespoon of cornstarch to ensure that the pie filling thickens.
This has to set before you slice it or it will be too juicy.