Make the crust. Combine the flour, sugar, and cubed butter in a large bowl. Drizzle in the water and use a pastry cutter, fork, or fingers to smash the butter into pea-sized crumbs. Then, work the dough to form a ball. If the dough is very dry, add in more water. You want to form a loose ball and flatten it into a disc shape. Work quickly to avoid melting the butter with your hands. Wrap the dough in plastic wrap and let the dough chill for an hour.
While the dough is chilling, prepare the fruit. Pit and chop the cherries and slice the nectarines. Toss with the sugar and cornstarch and set in the fridge until ready to use.
Prepare a surface with flour and get ready to roll out the dough. Use a rolling pin to roll the dough to about ¼-inch thick. The dough does not have to be in a perfect circle; it should aim for an even thickness. If you want even edges, use a pie or pizza cutter to slice off the edges for an even cut.
Spoon the fruit mixture into the galette, distributing it evenly. Leave about a 2-3 inch board between the fruit and the crust. Then, fold the crust over the fruit mixture, overlapping the edges as you go. Brush the edges with eggwash and top with the coarse sugar, if desired. Place the galette in the fridge to chill for 15 minutes.
While the galette is chilling, preheat the oven to 425.
Bake the galette for 30-35 minutes. When it is done, the galette will be golden brown, and the fruit will bubble.
Let the galette cool for about 10 minutes after it comes out of the oven so that all of the juices will not run out when it is cut.
