Place flour, sugar and salt in bowl of a food processor and pulse until combined. Add butter and pulse until mixture resembles coarse crumbs. Add water, one tablespoon at a time and pulse just until dough comes together (be careful to not over mix). Pat dough into a disk and wrap in plastic wrap. Chill at least 1 hour.

Preheat oven to 400°F. Roll out dough on a sheet of parchment paper to a 12-inch circle. Gently transfer dough and parchment paper to a large baking sheet.
 

In a large bowl, gently toss raspberries, blackberries, sugar, cornstarch, lemon juice and zest until evenly blended.

Spoon berry mixture onto dough circle, leaving a border of about 2 ½ inches . Fold edge of dough over berries. Gently crimp and pinch the fold to keep it from unrolling. If you wish, pinch the dough at even intervals to give it a decorative look. Brush edge of dough with egg white and sprinkle with sanding sugar. Bake about 40 minutes or until dough is golden brown and filling is bubbling. Let cool at least 15 minutes before serving. Just before serving, top with pistachios and dust lightly with confectioners’ sugar.