In a large bowl, whisk together flour, sugar, and salt. Cut in cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Add cold water gradually, mixing until the dough just holds together. Form into a disc, wrap in plastic wrap, and chill for at least 1 hour.

Roll the dough out to about a 12-inch circle. Using a 4-inch round cutter or bowl, cut out 6 smaller circles, re-rolling scraps as needed. Chill cut crusts for 15–20 minutes.

In a medium bowl, gently toss blueberries, strawberries, sugar, flour, lemon juice, and zest until evenly coated.

Place dough circles on a parchment-lined baking sheet. Spoon fruit filling into the center of each, leaving about a ½-inch border around the edges. Fold the edges over the filling to create a rustic pleated edge.

Place a cube of butter on top of the filling in each galette. Brush crust edges with beaten egg and sprinkle with coarse sugar.

Bake at 350°F (177°C) for 30 minutes or until the crusts are golden brown and the filling is bubbling. Let cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.