Preheat oven to 375°F (150°C). Line a baking sheet with parchment paper or a silicone liner.
Place the sliced apples in a large bowl. Add the brown sugar, lemon juice, flour, cinnamon, ginger, nutmeg, allspice, cardamom, and salt. Stir to combine.
Place the pastry dough on a lightly floured surface, and roll it into a circle about 13 inches (33cm) in diameter. It should be between 1/8- and 1/4-inch (0.3-0.6cm) thick. If you like, trim any excess dough to make a more uniform circle. Carefully transfer the dough onto the prepared pan.
Arrange the apples on top of the rolled crust, leaving a 2-inch (5cm) border all the way around the edge. Be careful not to transfer much of the liquid when transferring the apple mixture to the crust.
Carefully fold the edges of the crust about 2 inches over the filling. Work around the entire galette, pleating the dough as needed to secure.
Combine the egg and water to make an egg wash. Brush over the top crust. (You won’t use all of the egg wash.) Sprinkle the crust generously with coarse sugar, and also lightly sprinkle over the filling if you like.
Place the pieces of butter over the top of the filling.
Bake 35 to 40 minutes, or until the crust is golden brown and the filling is bubbly.
Cool the galette on the baking pan on a wire rack. Serve warm or at room temperature.
Video
Notes
Store in an airtight container in the refrigerator up to 4 days, although it will be best within a day or two.