To make the galette dough:
In a stand mixer using the paddle attachment, combine the flour, cornmeal, salt, and 1 tablespoon sugar. Add the cold butter all at once. Mix on low speed until the mixture begins to resemble coarse cornmeal with some larger chunks. Using a fork or your fingertips, add the ice water 1 tablespoon at a time, tossing it with the dough. After the fourth tablespoon has been added, squeeze a small amount of dough in your hand. If the mixture does not stay together and looks dry, add another tablespoon of water. If necessary, add even more water, drop by drop, until the dough just holds together. Dump the dough onto a piece of plastic wrap and gather it up to form a flat, round disk. Wrap it well in the plastic wrap and refrigerate it for at least an hour.
To make the filling:
Combine all of the filling ingredients in a large bowl and toss gently to combine.
To assemble and bake the dough:
Heat the oven to 400°F. Line a baking sheet with parchment paper. On a floured surface, roll out the dough into a 14- to 15-inch circle; trim any rough edges, if necessary, to maintain a round shape. Transfer the dough to the baking sheet, allowing any excess to hang over the sides. Pour the filling onto the dough, leaving a 2-inch edge of dough with no filling on it. Fold the dough over the fruit toward the center, pleating and overlapping as necessary. Brush the edges of the dough with the egg wash and sprinkle the galette with the 2 tablespoons sugar.
Bake until the crust is golden brown and the filling is bubbling, 45 to 50 minutes. Remove from the oven and let cool a little before cutting.