Preheat oven to 400°F. Line a cookie sheet with parchment paper.
Place the sliced apples in a large bowl and sprinkle them with lemon juice to prevent browning.
In a small bowl, mix the sugar with cinnamon. Sprinkle it over the apples. Stir in flour and toss to coat.
Lightly flour a flat surface and place the thawed puff pastry dough on top. Roll into a circle (14"D) or a rectangle.
Lift the puff pastry dough and carefully place on the prepared baking sheet.
Pour the apple mixture over the pastry dough, evening it out but stopping 2" from the edges.
Fold the edge of the puff pastry over some of the filling. Most of the apples will not be covered.
Whisk the egg and brush it over the edge of the puff pastry edge then sprinkle with coarse sugar.
Bake the galette for 20-25 minutes or until the puff pastry is golden brown.
Serve immediately topped with scoops of vanilla ice cream.
Notes
STORING
Short-Term Storage: If you plan to eat the apple galette within a day or two, you can store it at room temperature. Cover it with plastic wrap or store it in an airtight container.
Longer-Term Storage: For longer storage, it's best to refrigerate the galette. Cover it with plastic wrap or aluminum foil to prevent it from absorbing any odors from the fridge. This can extend its shelf life for up to 4 days.