In a food processor, combine flour, sugar, and salt. Pulse to combine. Add the butter and pulse a few times just to break the pieces down a bit.
While pulsing, drizzle in 4 tablespoons of ice water. Pulse until dough starts to clump together. Turn out dough onto a lightly floured surface and knead together to form a ball. Shape into a disk and tightly wrap in plastic wrap. Chill for at least 2 hours or up to 3 days.
For the Apple Filling:
Preheat the oven to 400°F. Line a baking sheet with parchment paper or a silicone baking mat.
Peel, core, and slice the apples into ⅛-inch thick slices. Place in a large bowl. Toss with lemon juice until well coated.
Sprinkle apples with sugar and cinnamon and toss until combined.
For the Assembly:
On a lightly floured surface, roll pie dough into roughly a 12-inch circle. Transfer to the lined baking sheet (the dough will likely hang over the edges.) Arrange the apple filling in the center of the dough, leaving a 3-inch border. Drizzle the apples with melted butter.
Fold the edges of the dough over the filling towards the center. (The dough will not fully cover the filling). Brush the dough with egg and sprinkle with sugar.
Bake for 45 minutes or until crust is golden brown and filling is bubbly. Let cool on a wire rack for at least 30 minutes before slicing.
Video
Notes
Don’t skip the final cooling before slicing the galette. While it is delicious eaten warm, it does need a bit of time to let the juices set before you cut into it. It will still be warm after 30 minutes!