To make the crust, combine the flour and salt in a medium bowl. Grate the cold butter directly into the flour, then stir to combine.
In a separate bowl, whisk together the egg, water, and vinegar.
Add the wet ingredients to the flour mixture and stir to combine. If the dough is too dry, gradually add water just until it clumps together. Wrap the disk of dough in plastic wrap and refrigerate it for at least 30 minutes.
Preheat the oven to 375°F. Place the chilled dough on a sheet of parchment paper and roll it out into a circular shape about 12 inches in diameter. Transfer the parchment paper with the dough onto a baking sheet.
To make the filling, toss together the cherries, sugar, and cornstarch in a large bowl until evenly coated.
Spoon the cherry mixture into the center of the dough, leaving a 2-inch border. Fold the border of dough over to enclose the fruit (it's okay if it overlaps).
To make the egg wash, beat the egg in a small bowl. Brush it onto the border of the crust, then sprinkle with sugar.
Bake the galette for 30-35 minutes, or until the crust is golden brown and the filling is bubbly. Allow the galette to cool for about ten minutes, then serve and enjoy!