Preheat the oven to 350f convection, or 375f regular. Begin by sifting the flour, sugar, salt, and pie spice into the food processor. Add the lemon zest, then pulse once or twice to combine.
Cube the cold butter, and add it to the food processor. Pulse several times until it looks like breadcrumbs.
Add the water bit by bit, pulsing between each addition. When the dough is starting to clump together, turn it out onto the worktop and form it into a ball with your hands.
Lay the pastry onto a sheet of baking parchment, and cover it with another sheet. Roll the dough out until it is about 1/8 inch thick, into a rough circular shape.
In a bowl, combine the apples, lemon juice, cornstarch and honey and stir well to combine. Remove the top sheet of baking parchment and lay the apples onto the pastry, leaving at least a 2 inch border free of fruit. Drizzle any remaining juices from the bottom of the bowl onto the fruit. Sprinkle with sea salt.
Fold the pastry border back onto the fruit to form a crust. Beat the egg with 1 tsp water, then brush this onto the pastry. Sprinkle with turbinado sugar and bake in the centre of the oven for about 30 minutes.
Rotate the tray halfway through cooking. The galette is done when the fruit is soft and the pastry is a deep golden-brown.
Serve with whipped cream if desired.