Prepare your dough. In a medium-sized mixing bowl whisk together 1 cup of all-purpose flour, salt, and 1 tablespoon sugar. Use your finger tops to mix the butter with the flour. Continue to mix until a coarse mixture forms and the butter is 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 the dough comes together.
Chill. Place the dough in a sealed zip lock bag and refrigerate for at least 30 minutes.
Prepare your filling. In a medium mixing bowl add the blackberries, blueberries, chopped rosemary and 1/3 cup sugar (this is an approximation- feel free to add a little less or a little more to suit your own personal sweet preference). Gently stir to combine then sprinkle with 1 tablespoon of flour. Mix again and set aside.
Roll your dough. But first, preheat your oven to 425 degrees F and line a large baking sheet with parchment paper. Then sprinkle a clean work surface with a small amount of flour. Roll the chilled dough into a circle(ish) shape, approximately 10-12 inches in diameter. It does not have to be a perfect circle.
Transfer and fill. Carefully transfer your rolled out dough to the lined baking sheet and scoop the blackberry mixture into the center making a small pile. Make sure to leave a 2-inch border around the edges.
Fold in the edges. With the filling in the center carefully fold the crust perimeter over the fruit filling toward the center (but not all the way), working your way around the circle and essentially creating a barrier for the berries to bake within. Pinch together the dough at the seams so it sticks together.
Gently brush the exposed dough with milk and sprinkle with any remaining sugar, if desired. Dot the fruit topping with butter (optional, but adds a nice, rich flavor).
Bake. Bake at 425 degrees for 5 minutes and then reduce the heat to 375 degrees F. Continue baking for an additional 25-35 minutes, or until the crust is golden and the filling is starting to bubble.
Cool and serve. Allow your baked galette to cool for 10 minutes before slicing, serving, and topping with a big scoop of vanilla ice cream if desired.