Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Prepare the berries by washing them gently and patting them dry. If using strawberries, hull and slice them into quarters or smaller pieces depending on size. Larger berries should be cut to roughly the same size as the smaller berries.
In a medium bowl, combine the berries, sugar, cornstarch, lemon juice, and zest. Toss gently to coat all the berries without crushing them. Let sit for 5 minutes to allow the sugar to start drawing out some juices.
Roll out your pie crust on the parchment-lined baking sheet to about a 12-inch circle. Don’t worry about creating a perfect circle—the irregular shape adds to the rustic charm.
Spoon the berry mixture onto the center of the dough, leaving about 2 inches of dough around the edges. Be careful not to include too much of the accumulated juice, as this can make the galette soggy.
Fold the edges of the dough up and over the berries, pleating as you go around. The center will remain open, showing off the colorful berries.
If using butter, dot the exposed fruit with small pieces to add richness and help create a glossy filling as it bakes.
Whisk together the egg and water to create an egg wash. Brush this mixture onto the folded dough edges.
Sprinkle the dough with coarse sugar for an extra crisp, sweet finish and beautiful sparkle.
Bake for 25-30 minutes until the crust is golden brown and the filling is bubbling. If the crust is browning too quickly, tent loosely with foil.
Let cool for at least 15 minutes before slicing to allow the filling to set slightly.