Whisk the flour, salt, and sugar together in a large mixing bowl.

Cut in the cold cubed butter with a pastry cutter or your fingertips until the mixture looks like coarse crumbs with pea-sized pieces of butter.

Mix the ice water with the vinegar, then sprinkle it over the flour a tablespoon at a time, tossing gently until the dough just holds together when pressed.

Divide the dough in two, shape each half into a disk, wrap in plastic, and chill for at least 60 minutes.

In a large bowl, toss the Saskatoon berries with the sugar, cornstarch, lemon juice, lemon zest, almond extract, cinnamon, and salt. Let it sit for 10 minutes while you roll the dough.

Roll one dough disk into a 30 cm / 12-inch round on a lightly floured surface and fit it into a 9-inch pie dish, trimming the overhang to 1 inch.

Pour the berry filling into the crust and scatter the cubed butter over the top.

Roll the second disk and cut it into 10-12 strips. Weave the strips into a lattice over the filling, then trim and crimp the edges.

Brush the lattice with the egg wash and sprinkle with coarse sugar. Chill the assembled pie for 15 minutes before baking.

Place the pie on a baking sheet to catch drips and bake at 220C/425F for 20 minutes.

Reduce the oven to 190C/375F and bake for another 35-40 minutes, until the crust is deep golden and the filling bubbles thickly through the lattice, about 55-60 minutes total.

Cool the pie on a rack at room temperature for a full 3 hours before slicing, so the filling has time to set.