4-6 tbs ice water
2 tsp cold cider vinegar
3 cups all-purpose flour, plus more for dusting
1 tsp salt
1 cup cold unsalted butter, cut into small pieces
8 oz cold cream cheese, cut into small pieces
In a large bowl, whisk together flour and salt. Cut in butter and cream cheese using a pastry blender until mixture resembles coarse crumbs, with pieces up to 1/2 inch.
Combine vinegar and ice water. Sprinkle over crumbs, stirring, until mixture just begins to hold its shape when squeezed. Divide crumbs in half and place each amount on a length of plastic film and wrap tightly.
Press wrapped dough into a disk using a rolling pin. Refrigerate for at least 1 hour.
On lightly floured surface, roll out the first disk to 13-inches. Fit into a 9-inch pie plate.
Filling:
7 generous cups frozen mixed blueberries, raspberries and blackberries
1/2 cup granulated sugar
1/4 cup cornstarch
1/4 tsp cinnamon
1 tbs fresh lemon juice
1 large egg yolk
1 tbs light cream
sugar for sanding
In a large bowl, toss frozen berries together with sugar, cornstarch, cinnamon and lemon. Allow to thaw. Stir and heap into pie shell.
Roll out second crust and fit over top. Trim and crimp or flute edges, then cut six 3-inch vents in crust.
Beat together yolk and cream. Brush over surface and edges of pie (some will be leftover: discard). Sprinkle generously with sugar. Chill for 30 minutes.
Preheat oven to 400F.
Transfer pie to a baking sheet and into oven. Bake 20 minutes, reduce temperature to 350F and bake 55 more.
Cool completely on a wire rack before serving.