Creating a Mixed Berry Galette is straightforward if you follow these simple steps:

1. Prepare the Dough: In a mixing bowl, combine the flour, granulated sugar, and salt. Add the cold, cubed butter. Use a pastry cutter or your hands to mix until the mixture resembles coarse crumbs.
2. Add Ice Water: Gradually add ice water, one tablespoon at a time, mixing until the dough comes together. Do not overwork it.
3. Chill the Dough: Form the dough into a disc, wrap it in plastic wrap, and refrigerate for about 30 minutes.
4. Preheat the Oven: Preheat your oven to 400°F (200°C) while the dough is chilling.
5. Prepare the Filling: In a bowl, combine the mixed berries, cornstarch, lemon juice, and brown sugar. Gently toss to coat the berries evenly.
6. Roll Out the Dough: On a lightly floured surface, roll out the chilled dough into a circle about 12 inches in diameter. Transfer it to a parchment-lined baking sheet.
7. Add the Filling: Spoon the berry mixture into the center of the dough, leaving about a 2-inch border around the edges.
8. Fold the Edges: Gently fold the edges of the dough over the filling, pleating as you go to create a rustic look.
9. Make the Egg Wash: In a small bowl, whisk together the egg and milk. Brush the edges of the dough with the egg wash.
10. Bake: Sprinkle the top of the galette with brown sugar and bake in the preheated oven for 35-40 minutes, or until the crust is golden brown and the filling is bubbling.
11. Cool: Allow the galette to cool for about 10 minutes before slicing.

Following these steps will guide you in creating an incredible Mixed Berry Galette that is sure to impress!