Add the egg and process just until combined. With the processor on low, drizzle in the ice water just until the dough starts to clump.
1 large egg, 2-3 Tablespoons ice water
Then, turn the dough out onto a very lightly floured board and just gently pull it together – don’t knead it too much. Wrap it in plastic wrap or baking paper and place it in the fridge for ½ an hour to rest.
Making the filling
Preheat the oven to 395°F and line a baking tray with parchment paper.
Peel the apples, then chop each apple into quarters. Slice out the core of each quarter, and then cut each quarter into 4 or 5 slices. The result should be thin, even slices of apple.
3 medium granny smith apples, ¼ cup packed light brown sugar
Place the apple slices in a bowl and add the sugar, flour, cinnamon and lemon juice. Toss so all of the apples are evenly coated.
1½ Tablespoons all-purpose flour, 2 teaspoons cinnamon, squeeze of lemon
Assembling the galette
Flour a clean surface. Place the dough on top, then sprinkle the top of the dough with a little flour, too. Using a rolling pin, gently roll out the dough to about a 10-inch circle (about ½ of an inch thick).
Place the rolling pin on the dough on the furthest side from you. Wrap the top of the dough over the rolling pin and roll it back towards you so the dough rolls around the rolling pin.
Roll it back out in reverse onto the prepared baking tray.
Arrange the apples in the center of the disk of dough, making sure to leave an inch and a half (1½) of space clear around the edge. Discard any remaining juice that may have come out of the apples in the bowl.
Gently gather the edges of the dough and fold over the edge of the apples, leaving the center uncovered.
Brush the egg around the edge of the pastry and sprinkle with the turbinado sugar.
1 egg, 2 teaspoons turbinado sugar
Bake for around 45 minutes until the edges of the pastry are golden and crisp. If there are any pools of liquid in the galette, you can just soak a little away with paper towel. Allow to cool for 5 minutes before serving. Enjoy!
Notes
The water and butter need to be COLD: After you chop the butter, you may want to put it in the freezer. Both the water and butter should be ice cold for the crust.
Don't knead the dough too much: It's important not to play with the dough or knead it too much. You want the cold butter to stay cold and not melt into the dough for a crisp, flaky pastry.
Be careful not to overstuff: Too much filling may may make the galette bake improperly. Carefully follow the directions so you don't overstuff your galette.
Put a dab of butter on top: Adding a pat of butter to the top of the galette while it bakes will add an extra rich layer to an already delicious dessert!