To make the crust, whisk the flour, sugar, and salt together in a large bowl. Cut in the butter using a pastry blender (or two knives) until the mixture is crumbly. Add the ice water, one tablespoon at a time, and stir with a rubber spatula until dough starts to come together.
Form the dough into a round disc and wrap tightly in plastic wrap. Chill in the refrigerator for at least 1 hour.
Preheat the oven to 375°F. Line a baking sheet with parchment paper and set aside.
To make the filling, toss the berries with the sugar and arrowroot in a large bowl. Allow the mixture to sit at room temperature while you roll out the dough.
Transfer the dough to a lightly floured surface. Roll the dough out to a large circle, about 12" in diameter. If the dough is still crumbly, sprinkle ice cold water onto the surface and work it into the dough until it comes together. Carefully transfer the dough to the prepared baking sheet.
Spoon the filling onto the center of the dough, being careful not to transfer the liquid. Leave a 2" border around the edges. Gently fold the edges of the dough over the fruit and crimp as necessary to seal. Brush the edges with half and half, if using. Bake for 40-45 minutes, or until crust is slightly golden. Allow the galette to cool on the baking sheet for at least 15 minutes before cutting and serving.
Notes
Cornstarch can be used as a 1:1 substitute for arrowroot.
I chose not to brush the tops with egg wash because I don't use eggs in my baking. In this recipe, the half and half serves as an egg wash substitute. If you are not allergic to eggs, you can go ahead and use egg wash to achieve a more golden color. If you do not have half and half, heavy cream may also be used.