Put 2 cups flour, granulated sugar and 1/4 teaspoon salt in the bowl of a food processor and pulse briefly to combine.

Add butter and pulse until mixture resembles coarse meal. (Alternatively, cut butter in by hand with two table knives.)

Add 1/4 cup ice-cold water and pulse, or mix by hand with a fork, just until dough begins to come together.

Form dough into two 4-inch discs, cover and chill until firm, about 2 hours or overnight.

Preheat the oven to 350°F.

Put blackberries, raspberries, blueberries, 1/2 cup of the raw sugar, remaining 1/2 cup flour, zest and a pinch of salt in a large bowl and mash together gently until well combined and juicy but still chunky; set aside.

On a lightly floured surface, roll out one piece of dough into a 10-inch circle and gently transfer to an 8-inch pie pan.

Transfer berry mixture to pie pan; set aside.

Roll out remaining dough into a 10-inch circle and transfer to pie pan, gently draping it over the berry filling.

Firmly press the edges together and then trim away excess dough.

Pinch or crimp edges as desired and cut 3 slits in the top of the pie to allow steam to escape.

Brush crust with milk and sprinkle with remaining 1 tablespoon raw sugar.

Bake until golden brown and bubbly, about 1 hour. Set aside to let cool completely then serve.