Preheat oven to 375°F (190°C).
In a bowl, mix flour, salt, and sugar for the crust. Cut in cold butter until mixture resembles coarse crumbs.
Gradually add ice water until dough forms. Wrap in plastic and chill for at least 30 minutes.
Toss mixed berries with sugar, cornstarch, and lemon juice.
Roll out chilled dough into a rough circle on parchment paper.
Place berry filling in the center, leaving a 2-inch border. Fold edges over the berries, pleating as needed.
Brush crust with egg wash if desired.
Bake for 40–45 minutes until crust is golden and berries are bubbling.
Cool slightly, dust with powdered sugar, slice, and serve.
