Whisk the flour, sugar, and salt together in a medium bowl. Using a pastry cutter (love this one) or a couple forks, cut in the butter until the mixture resembles coarse pea-sized crumbs. Add the water and stir until the flour is moistened. Add 1 more Tablespoon of water if the dough seems dry. 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 and up to 3 days (or freeze up to 3 months).

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 until step 6.

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 until step 6.

Preheat oven to 425°F (218°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.

On a lightly floured work surface, roll the dough into a 12-inch circle. If desired, trim the rim of the circle to make a clean edge. Transfer dough to the prepared baking sheet. (You can also roll the dough right out onto the parchment paper or silicone baking mat that you are using to line the baking pan. If doing so, lightly flour the parchment paper or silicone baking mat. This is what I usually do.)

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. Press gently to seal the edges. Brush the crust edges with egg wash and sprinkle the crust with sliced almonds and coarse sugar, if desired.

Refrigerate the shaped galette for at least 20 minutes and up to 8 hours. If refrigerating for longer than 1-2 hours, cover it lightly.

Bake until the filling is bubbly and the crust is golden brown, about 35-38 minutes. Cool on the baking sheet for 10 minutes before slicing and serving. Tastes wonderful served warm with vanilla ice cream!

Cover and store leftover galette in the refrigerator for up to 5 days.