First prepare the dough. Whisk together 1 cup of the all-purpose flour, salt and 1 tablespoon sugar together in a medium size bowl. Add the chopped butter and, working with your fingers, break up the butter in with the flour until a course mixture is formed.Add the chopped butter and, working with your fingers, break up with butter in with the flour until a course mixture is formed with the butter broken down to the size of small peas. Add the iced water, three tablespoons to start, and more (1 tablespoon at a time) as needed. Gently mix until dough comes together.
Place the dough in a sealed zip lock bag and refrigerate for at least 30 minutes.
Meanwhile, prepare the filling. In a medium mixing bowl add the blackberries, blueberries, chopped rosemary and 1/3 cup sugar (approx. feel free to add a little less or a little more to suit your own personal sweet preference). Stir to combine. Sprinkle the flour over the blackberry mixture and stir to combine. Set aside.
Preheat oven to 425 degrees F.
Sprinkle a clean work surface with a small amount of flour. Roll the chilled dough into a circle into a large circle approximately 10-12 inches in diameter.
Line a large baking sheet with parchment paper.
Carefully transfer rolled out dough to the lined baking sheet and scoop the blackberry mixture into the center making a pile, but making sure to leave a 2-inch border around the edges.
With the filling piled in carefully start folding in the dough toward the center, working your way around the circle and essentially creating a barrier for the berries to bake within.
Gently brush the exposed dough with milk and sprinkle with any remaining sugar, if desired.
Bake at 425 degrees for 15 minutes and then reduce the heat to 375 degrees F. Continue baking for an additional 30-35 minutes, or until the crust is golden and the filling is starting to bubble.
Allow the baked galette to cool for 10 minutes before serving.