In a large bowl whisk together the flour and salt.
Add the butter to the flour. Toss the butter with the flour and start to squeeze the butter into the flour until you have some large and some small flattened pieces of butter.
Add 5 tablespoons of the water to the mixture and toss gently to combine, start to bring the dough together.
If the dough seems dry and is not holding together add additional water 1-2 teaspoons at a time and continue to gently mix until the dough until it holds together.
Gather the dough together and form into a disc about 1" thick.Wrap in plastic and refrigerate for 1-2 hours.

In a small bowl whisk together the sugar, cornstarch, cinnamon and salt.
Add the blueberries and blackberries to a large saucepan, along with lemon juice and water.
Heat on medium until the berries start to release their juices. Add the sugar mixture and mix all together.
Allow the filling to cook over medium low heat, until it thickens and begins to bubble. Let it bubble for about a minute, stirring occasionally so the filling does not stick to the bottom of the pan. It may take any where from 6- 8 minutes.
Remove from the heat and add the raspberries. Set aside to cool. (Raspberries tend to break down very quickly, so adding them in after the filling is cooked will help them to retain their shape.)
Assembly:
Remove the chilled disc of dough from the fridge and place it onto a lightly floured surface.
Roll it into a 14” circle and place it on a parchment lined baking sheet.
Place the cooled filling in the center of the dough leaving about a 2” border
Fold up the sides over the filling, egg wash the exposed crust and sprinkle with sugar.
Preheat your oven to 400 F.
Place the galette in the freezer to chill for at least 30 minutes.
Place the galette on the bottom rack of your oven and bake for 15 minutes.
Reduce the temperature of the oven to 375 F and bake for an additional 30-35 minutes or until the crust is golden brown and the filling is bubbling. Check the bottom of the crust to make sure that it is fully cooked.
Allow to cool completely.
Enjoy with whipped cream or ice cream!