Prepare the crust - In a large bowl, mix together flour, 1 tablespoon of sugar, and salt. Cut butter into small cubes and add to the bowl. Use your hands or a pastry cutter to break the butter into the flour until the mixture becomes crumbly. Add 4-6 tablespoons of ice water, one tablespoon at a time, and knead until the dough comes together. Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes. - Make sure the butter is very cold, and handle the dough as little as possible. The key to a flaky crust is the chunks of butter in the dough that will melt in the oven and create pockets of air.

Make the filling - In a separate bowl, mix together the berries, remaining sugar, cornstarch, and vanilla extract. Set aside. - Make sure the fruit is dry before mixing it with the cornstarch and sugar. You can wash and pat dry the fruits with a paper towel.

Roll the dough - Preheat your oven to 375°F (190°C). Roll the dough into a 12-inch circle and transfer it to a baking sheet lined with parchment paper. - Flour your work surface generously and use a rolling pin to roll the dough evenly. To transfer the dough without tearing, fold it in half and then in half again, then place it on the baking sheet and unfold it.

Add the filling - Pour the berry mixture onto the center of the dough, leaving a 2-inch border around the edges. Gently fold the edges of the dough over the filling, pleating as you go. - Use a slotted spoon to transfer the fruit to the center of the dough, leaving any collected juices behind. The fruit should be arranged in a circular pattern.

Bake the galette - Beat the egg in a small bowl and brush the edges of the dough with it. Sprinkle the edge with turbinado sugar. Bake the galette in the preheated oven for 45 minutes or until the crust is golden and the filling is bubbly. Remove from oven and let cool on a wire rack. Serve hot or cold. - You can use any leftover egg to create an egg wash for other recipes. A pastry brush works better when it comes to brushing the egg onto the dough edges than your fingers.