Preheat oven to 400°F.
Stir together sugar, lemon juice, zest, and cornstarch until just blended. Fold in berries gently until they are covered with the lemon mixture.
Line a pie pan with one of the crusts, and spoon berry filling into the pan. Cover with second crust, and crimp the edges together.
Whisk together egg yolk and water in a small bowl. Brush the top of the pie with the egg wash using a pastry brush, and cut slits into the top of the pie. Bake for 40-45 minutes, or until the berries are bubbling and the crust is golden brown.
Serve with fresh whipped cream or vanilla ice cream, or just enjoy it plain