Combine dry ingredients. In a large bowl, whisk together the flour, salt, and sugar until evenly combined.
Incorporate the butter. Add the cold butter cubes and use a pastry cutter to work them into the flour mixture until coarse crumbs form.
Add the water. Gradually add the ice water, mixing gently until the dough just comes together when pressed. Avoid overmixing to keep the crust tender and flaky.
Chill the dough. Form the dough into a flat disk, wrap tightly, and refrigerate for at least 1 hour before using.
Roll the dough. On a lightly floured work surface, roll the chilled dough into a 12-inch circle. Transfer the dough to the prepared baking sheet.
Prepare the filling. In a large bowl, gently toss to combine the berries, sugar, cornstarch, and lemon juice.
Assemble. Add the filling into the center of the dough, leaving about a 2-inch border all around.
Fold. Gently fold the edges of the dough over the fruit, overlapping the dough as necessary. Press gently to seal the edges. Brush the edges with egg wash and sprinkle with coarse sugar.
Bake. Bake at 400°F until the filling is bubbly and the crust is golden brown, about 25 to 30 minutes.
Cool. Remove from the oven and cool on the baking sheet for 10 minutes.
Serve the galette warm with ice cream or enjoy it cooled.