In a small bowl. whisk together the egg, vinegar, and water. Set aside.
In a food processor, pulse together flour, sugar, and salt a few times until well combined.
Gradually add the cold cubed butter, pulsing until the dough is crumbly and butter is well combined.
Slowly add the egg/vinegar/water mixture and pulse to combine just until the dough forms.
Carefulyl remove the dough from the food processor and place onto plastic wrap. Shape into a disc, cover completely with plastic wrap, then chill in the refrigerator for at least 30 minutes and up to 3 or 4 hours.
After chilling, remove from the refrigerator and set out onto the kitchen counter or other flat surface.
Preheat the oven to 400°F. Cut out a piece of parchment paper big enough to fit on top of a baking sheet.
Make the filling
In a large bowl, toss together the pitted cherries, sugar, cornstarch, salt, and vanilla until well combined.
Assemble the galette
Onto the prepared piece of parchment paper, roll out the dough into a circle about ¼" thick.
Place the cherry filling mixture in the center of the dough circle, leaving about 2 inches of space all around.
Fold in the edges of the dough, slightly overlapping some parts for that classic galette look (see step-by-step photos for example).
In a small bowl, whisk together the remaining egg and water to form an egg wash.
Using a basting brush or pastry brush, brush egg wash evenly over the edges of the dough.
Sprinkle remaining cold butter cubes over the top of the filling, spread out evenly across the filling.
Sprinkle sanding sugar over the dough and the filling.
Very carefully transfer the parchment paper with pie onto a baking sheet (you might need an extra pair of hands for this!).
Bake at 400°F for 40-50 minutes, until the edges of the dough are a beautiful lightly golden color.
Notes
Makes approximately 8 servings. Storage: allow the pie to cool down, then wrap with plastic wrap and store in the refrigerator for up to 4 days. Reheat in the microwave or enjoy cold.