Make the crust: Using a food processor: Whisk the flour, sugar, and salt together. Add chopped butter and coarsely mix.. Add the water and mix until the flour is moistened. Turn the dough out onto a lightly floured work surface and, using your hands, work the dough into a ball. Flatten it into a thick disk. Wrap the dough disk in plastic wrap and refrigerate it for at least 1 hour.
As the dough chills, prepare the frangipane: Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the frangipane ingredients in a medium bowl on medium-high speed until combined and creamy. Cover and refrigerate
Prepare the blueberry peach filling: Gently toss the blueberry peach filling ingredients together in a medium bowl until all the fruit is coated with almond flour and sugar. Cover and refrigerate
Preheat oven to 425°F (190°C). Line a large baking sheet with parchment paper or a silicone baking mat
On lightly floured parchment paper, roll the dough into a 12-inch circle
Evenly spread the frangipane into the center of the dough, leaving a 3 inch edge. Spoon or arrange the peaches and blueberries on top of the frangipane. Leave any fruit juices that are at the bottom of the bowl—that could make the galette filling too soggy. Gently fold the edges of the dough over the filling, overlapping the dough as necessary. Brush the crust edges with egg wash and sprinkle the crust with sliced almonds and coarse sugar.
Lightly cover and refrigerate the shaped galette for at least 20 minutes and up to 8 hours.
Bake until the filling is bubbly and the crust is golden brown, about 35-40 minutes. Cool on the baking sheet for 10 minutes before slicing and serving.
