Mix the dough
In a food processor, combine the flour and salt and pulse briefly to mix. Scatter the butter over the top and pulse just until the mixture forms coarse crumbs about the size of peas. Drizzle the ice water over the flour mixture and pulse just until the dough starts to come together.
Transfer the dough to a work surface, divide into 3 equal pieces and press each into a flat disk. Set 1 disk aside and wrap the remaining 2 disks in plastic wrap for future use (see note below).
Preheat an oven to 425°F. Use middle oven rack. Line a baking sheet with parchment paper.
Place the dough on a lightly floured surface and roll out into a 13-inch round. Fold the round in half, transfer to the prepared sheet and unfold the round.
Fill and bake the galette
In a small bowl, mix the sugar and flour together. In a medium bowl, add the berries, lemon zest and lemon juice. Toss the sugar/flour mixture in with the berries and stir lightly until well mixed. Spoon the filling onto the dough, leaving a 2-inch border uncovered around the edge. Fold the edge up and over the filling, forming loose pleats. Using a pastry brush, lightly brush the crust with egg white, then sprinkle sugar over the crust and top of the berries. Now dot the 2 teaspoons of butter cubes (or grated butter) over the top of the fruit. You don't need a lot of this, but doing so will help the fruit achieve a nice sheen after baking.
Put the galette in the 425F degree oven and immediately turn the oven down to 400F degrees and bake for 25 minutes, or until the filling is bubbling and the pastry is golden brown. (During last 5 minutes, if the crust is really brown before the filling bubbles, place a piece of aluminum foil over entire galette until the end of baking so crust doesn't burn.)
Transfer the galette to a wire rack and let cool slightly. Cut into wedges and serve.
Cook's note: the two extra disks of dough are a bonus! Use to make other fruit galettes quickly during the week. Extra disks can remain in fridge for up to 5 days, or refrigerate for up to 2 months. I found the disk that had been refrigerated overnight was even more flaky than the one I made immediately.
Extra tip if your fruit filling is very wet, like juicy mid-summer strawberries- Before piling on the fruit, dust the galette dough with just a tiny bit of semolina flour or breadcrumbs before adding the fruit. This will ensures the crust won't get soggy.