Preheat oven to 375º. When the dough has chilled at least 30 minutes, remove from the fridge and lightly dust a piece of parchment paper with flour (the dough will stay on this parchment paper for baking, so it can be as big as the cookie sheet you’ll bake on).
Roll out the dough to ¼-inch thick and place the parchment paper and dough on the baking sheet you’ll use in the oven.
Place cherry filling mixture in the center of the dough, leaving a 2-3 inch border.
Fold the edges of the dough in, layering sections as you go and pressing down to seal. Make sure there are no cracks for juice to leak out.
Whisk one egg and brush the exposed dough with the egg wash. Bake for 45-55 minutes (check at 40) to estimate remaining time), until the fruit mixture is bubbling and the pastry is golden brown. If the crust starts to brown too much, loosely tent the galette with a piece of foil and continue baking until its done. Remove the foil as soon as it comes out of the oven.
Let cool at least 30 minutes, then serve with whipped cream or ice cream. Excellent at any temperature.