In a large bowl, whisk together the flour, sugar, and salt. Cut in the cold, cubed butter with a pastry blender or fork. Blend until the butter is completely coated in the flour mixture and resembles crumbs about the size of a pea.

Slowly add in the water, 1 tablespoon (15 ml) at a time, until the crust just comes together with your hands. It should still be a bit crumbly. Knead the dough until it is more consistent, being careful not to over handle it and melt the butter. Shape the dough into a round disc and wrap it in plastic wrap. Refrigerate the dough for at least 1 hour or up to 2 days.

In a medium bowl, combine the blueberries, strawberries, sugar, flour, lemon juice, and lemon zest. Stir until everything is well coated, about 2 minutes.

Roll out the pie crust to a 12-inch circle. With a 4-inch round cookie cutter or a small bowl, cut out 6 circles. You will have to knead the dough back together 1 or 2 times to cut out all of the circles. Line a baking sheet with parchment paper and place the pie crusts on the baking sheet. Place the crusts into the refrigerator for about 15 to 20 minutes to chill them.

Beat the egg in a small dish. Bring the crusts back out to the counter and brush them all over with the egg wash.

Divide the fruit filling into 6 equal portions and place it in the center of each galette. Then, fold the dough up around the filling, not completely covering the fruit. It should look rustic and a bit rough. Pinch any seams together so that the crust does not unfold while baking. Repeat this folding with all of the galettes. Brush the outside of the crust with egg wash. Place the 6 pieces of butter on top of the fruit. Sprinkle the outside edge of the galettes with the coarse sugar.

Bake the galettes for about 30 minutes or until the crust is golden brown and the fruit filling is thickened and bubbly. Let them rest on the countertop for about 10 minutes before serving. They can be served alone or with ice cream or whipped cream!