In a food processor, add the flour, salt, and sugar,. Pulse to mix.
Add the cubed butter and pulse until the mixture resembled coarse crumbs. Run the food processor on low and drizzle in the ice water just until it comes together.
Turn the mixture onto plastic wrap and shape into a large disc. Tightly wrap and chill for at least an hour, preferably overnight.
Preheat the oven to 350 degrees F.
Place the dough on a lightly floured surface and roll out into a large disc that is about ¼ inch thick. It is okay if the edges are imperfect - it adds to the character!
Mix together the plums, sugar, cornstarch, cinnamon, ginger, zest, salt, and vanilla together until well combined.
Arrange the plums on the dough working from the center outward. Leave a border around the plums. Fold the border over the plum slices.
Brush the egg wash on the folded over edge of the galette.
Gently transfer the galette to a baking sheet. Bake for 35 - 40 minutes, or until the crust is golden brown and the plums are cooked through.
Notes
Do not be tempted to load on extra layers of plum. They are very juicy and the juices will soak through the galette dough leaving you with a soggy mess.
You don't need to peel the skin of the plums if you don't want to.
Leave the dough out for about 5 - 10 minutes before rolling out to help prevent tons of cracks in the dough. If cracks do form, you can use your hands to press them back together.
Once you've rolled out the galette dough, transfer it to your baking sheet before adding the plums.
Leave a border of about 1 ½ - 2-inches to fold over the plums.