- 1. Mound flour on a large work surface and toss with butter. Using the heel of your hand, press the butter into the flour. With a pastry scraper or off-set spatula, scrape up the butter and flour mixture and turn it back into the pile. Keep working the dough until the butter is pressed into flakes and the flour starts to come together into a shaggy mass. Make a well in the flour mixture and add the cold water. Knead the water into the dough until it just comes together. Wrap the dough in plastic and refrigerate for 30 minutes.
- 2. Preheat oven to 350 degrees and line a baking sheet with parchment. Toss the sliced peaches with the peach preserves.
- 3. Lightly flour your work surface and roll out the dough into a 13-inch circle, about ⅛-inch thick. Transfer the dough to your baking sheet. Spoon the peach mixture onto the dough, leaving about a 2-inch border around the edges. Spoon the blackberries over the peaches. Fold the remaining dough in to form a crust and enclose the fruit. Brush the crust with the egg, then sprinkle with coarse sugar.
- 4. Bake the galette until the crust is golden brown and the filling is bubbling, about 45-60 minutes.
- 5. Cut into slices, and serve with ice cream, if desired.