In a large bowl, whisk together the all-purpose flour and salt. Add the cold, cubed butter and cut it into the flour using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Gradually add the ice water, 1 tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix. Form the dough into a disc, wrap it tightly in plastic wrap, and chill in the refrigerator for at least 30 minutes.

While the dough chills, prepare the filling. In a separate large bowl, gently combine the pitted Saskatoon cherries, granulated sugar, cornstarch, vanilla extract, and lemon juice. Mix well until the cherries are evenly coated.

Preheat your oven to 375°F (190°C). Line a large baking sheet with parchment paper.

On a lightly floured surface, roll out the chilled dough into a rough 12-inch circle. Carefully transfer the rolled dough to the prepared baking sheet.

Spoon the Saskatoon cherry filling onto the center of the dough, leaving a 2-inch border around the edges.

Carefully fold the edges of the dough over the filling, pleating as you go, to form a rustic crust around the cherries.

Brush the folded dough edges with the beaten egg wash and sprinkle generously with coarse sugar.

Bake for 45-55 minutes, or until the crust is golden brown and the cherry filling is bubbly and thickened. If the crust browns too quickly, you can loosely tent it with aluminum foil.

Remove the galette from the oven and let it cool on the baking sheet for at least 30 minutes before slicing and serving. This allows the filling to set properly.