Perfect Blackberry Pie - cooking recipe

Ingredients

classic pie crust (see Classic Pie Crust, Idiot Proof Step-By-Step Photo Tutorial)
5 1/2 cups fresh blackberries
3/4 cup granulated sugar, plus
2 tablespoons granulated sugar
1 pinch salt
1/4 cup potato starch or 1/4 cup potato flour
1 teaspoon finely grated lemon, zest of
1 tablespoon butter, cut into small pieces
1 egg white, beaten,for pastry wash
1 1/2 tablespoons granulated sugar

Preparation

Adjust oven rack to lowest position and place four 9-inch unglazed quarry tiles on rack to form an 18-inch square.
(A pizza stone will work, too.) Preheat oven to 400 degrees and let tiles heat for at least 30 minutes.
Puree 1 cup of the blackberries in a food processor fitted with the metal blade; set aside.
Stirring constantly with a rubber spatula, bring the sugar, salt, starch and blackberry puree to the consistency of thick jam over medium-low heat in a large saucepan.
Stir in lemon zest.
Pour thickener over remaining blackberries and stir to combine.
Pour blackberry filling into bottom pie shell and dot with the 1 Tb.
butter.
Cover with top layer of pastry dough.
Set an 18-inch square of heavy-duty foil on tiles or pizza stone.
Set pie on foil and bake for 20 minutes.
Remove pie from oven, brush with beaten egg white, sprinkle with 1 1/2 Tbs.
sugar.
Bake until golden brown, about 20 minutes longer.
Bring foil around pie to loosely cover.
Bake until filling bubbles, 15-20 minutes longer.
Cool on a wire rack, 2-3 hours.