Preheat the oven to 350 degrees . In a large bowl, toss together the frozen berries, sugar, cornstarch and lemon juice.
Unroll a pie crust into a 10-inch pie plate. Using your thumbs, gently press the pastry up the sides of the dish so it is even with the top rim.
Pour the fruit mixture into the crust and dot with the butter. Moisten the rim of the pastry with water. Unroll the second pie crust, set it on top of the berry mixture and pinch the crusts together with your fingers to seal. Cut 3 small slits in the center of the crust.
Mix together the egg yolk and cream and brush onto the pie. Set the pie on a baking sheet and bake for 30 minutes; increase the heat to 375 degrees . and bake until the crust is nicely browned, about 20 minutes longer.