Combine the flour with the butter, sugar and salt in a food processor and process until mixture resembles coarse meal with a few pea-size pieces remaining.
Add the ice water over the flour mixture and process until the pastry just begins to come together.
If not using a food processor, you can make the pastry by hand with a pastry cutter.
Pat dough into approx a 6" disk. Wrap in plastic and chill at least 1 1/2 - 2 hours. This can be made well ahead of time if necessary.
Combine berries, sugar and cornstarch in a mixing bowl. Gently mix together.
When dough is chilled, preheat oven to 400°F
On a floured surface, roll out dough until roughly a 12"x14" rectangle. The corners can be rough and uneven and it's fine if you can't get roll the dough out to the full measurement.
Transfer rolled dough onto a cookie sheet lined with parchment paper.
Brush about half of the egg white onto the dough.
Spoon the berry filling onto the dough, leaving a 2" border uncovered around the edge.
Fold the edges up and over the filling, overlapping slightly.
Brush remaining egg white over dough and sprinkle with turbinado sugar.
Bake until the filling is bubbly and the pastry is golden brown & crisp - about 40 - 50 minutes.
If any of the berry mixture oozed out, spoon on top of galette
Let rest on cookie sheet for about 10 minutes and then transfer galette, still on parchment paper, to wire rack to cool.
Slice & enjoy ♥