Flour the dough and your work surface then roll out dough, into a flat 12"/30cm wide circle using downward pressure (rather than outward).
Transfer onto a piece of parchment then continue to roll out into a 13-14"/33-35c wide circle.
Lay apples onto galette dough and pleat crust as shown in video @__6:45__ (or dump them all in the center if you want to be less fussy).
Brush visible crust around the perimeter of dough with egg wash then sprinkle egg washed crust with sugar in the raw.
Drizzle in a few spoons of the cinnamon corn starch slurry that the apples were sitting in.
Sprinkle with a pinch of flaky salt and place 3-4 slices of butter on the apples.
Transfer galette and parchment onto a baking sheet and bake at 375°F/190°C for 40-50 minutes, rotating pan halfway through.