Combine flour, granulated sugar, and salt in a large bowl. Cut in the cold butter until the mixture contains pea-sized pieces of butter.

Add 6 tablespoons ice water and gently mix. Add additional ice water one tablespoon at a time until the dough holds together when pressed.

Divide the dough into two disks, making one slightly larger. Wrap and refrigerate for at least 30 minutes.

Preheat oven to 425°F (220°C). Combine Saskatoon berries, sugar, cornstarch, lemon juice, lemon zest, cinnamon, salt, and almond extract if using.

Roll the larger disk of dough into approximately a 12-inch circle and transfer it to a 9-inch pie dish.

Pour the Saskatoon berry filling into the prepared crust and spread evenly. Dot the filling with 1 tablespoon of butter.

Roll out the remaining dough and place over the filling. Trim and crimp the edges, then cut several steam vents in the top crust.

Whisk the egg with 1 tablespoon water. Brush over the pastry and sprinkle with coarse sugar if desired.

Place the pie on a baking sheet and bake for 20 minutes at 425°F (220°C). Reduce oven temperature to 375°F (190°C) and bake for another 35 minutes until the crust is golden and the filling bubbles.

Transfer the pie to a wire rack and cool for at least 3 hours before slicing so the berry filling can fully set.