In a large bowl, whisk together the flour, sugar, and salt.
Add the cold butter. Use a pastry cutter, fork, or your fingertips to work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Add the ice water one tablespoon at a time, stirring gently until the dough begins to come together. Avoid adding more water than necessary.
Turn the dough onto a lightly floured surface and gently press it into a disk. Wrap and refrigerate for at least 20 minutes.
While the dough chills, prepare the fruit filling. Add the strawberries, cherries, sugar, cornstarch, lemon juice, lemon zest, vanilla, and salt to a large bowl. Toss gently until the fruit is evenly coated.
Preheat the oven to 400°F. Line a large baking sheet with parchment paper.
Place the chilled dough on a lightly floured surface and roll it into a circle approximately 12 inches in diameter.
Carefully transfer the dough to the prepared baking sheet.
Spoon the strawberry cherry filling into the center of the dough, leaving a 2-inch border around the edges. Leave behind any excessive liquid that has collected at the bottom of the bowl.
Fold the edges of the dough up and over the fruit, pleating the dough as you work around the galette. The center should remain open.
Whisk the egg with one tablespoon of water. Brush the egg wash over the exposed crust. Sprinkle the crust with coarse sugar, if desired.
Bake for 40–45 minutes, or until the crust is deeply golden and the fruit filling is bubbling.
Allow the galette to cool on the baking sheet for at least 20 minutes before slicing. The filling will thicken as it cools. Serve with vanilla ice cream or whipped cream, if desired.