In a large bowl, mix together the apple slices, sugar, flour, spices, and vanilla. Cover tightly and set aside.
Preheat the oven to 200C/400F/Gas 6.
On a lightly floured surface, roll out the disc of dough to a 12-inch circle. Transfer to a baking tray lined with parchment paper or a silicone mat.
Arrange the apple slices (discard the juices) in the centre of the dough, leaving a 2 - 3-inch border around them. Fold the edges of the dough over the apples, gently sealing any overlaps. Brush the crust with the egg wash, and sprinkle with sugar.
Bake for 35 - 45 minutes or until the filling is tender and bubbling, and the crust is golden brown. Allow to cool for about 10 minutes on the baking tray before slicing. Serve warm or at room temperature.
Notes
Leftovers can be stored, covered tightly, in the fridge for up to 5 days. Baked galette freezes well for up to 3 months. Thaw overnight in the fridge. Serve warm.Both the dough and filling can be made ahead of time, covered tightly and refrigerated for up to 3 days.