Steps:

• Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with aluminum foil.
• Mix flour, 1/2 cup sugar, baking powder, and salt in a bowl; mash butter into the flour mixture until crumbly. Stir boiling water into the flour mixture to form a dough.
• Dissolve cornstarch in cold water in a separate large bowl. Stir blueberries, raspberries, 3/4 cup sugar, and lemon juice into the cornstarch mixture; transfer to a cast-iron skillet and bring to a boil, stirring frequently. Drop dough into the skillet by the spoonful. Put skillet onto the prepared baking sheet.
• Bake in preheated oven until the topping is golden brown, about 25 minutes.

Steps:

• Preheat oven to 400 degrees F (200 degrees C).
• Combine flour and salt in a bowl. Mash shortening into the flour mixture with a pastry cutter or fork until the mixture is crumbly. Gradually add cold water as needed, tossing with a fork until dough forms a ball. Wrap dough in plastic wrap and refrigerate for 30 minutes.
• Turn dough out onto a flat work surface dusted with flour. Divide dough in half and form each half into a ball. Roll dough ball from center to edges into a circle 2 inches larger than the pie plate and about 1/8 inch thick. Wrap dough around rolling pin, starting on one side of the circle. Unroll over pie plate and trim dough. Roll remaining dough ball into a crust to fit the top of the pie and set aside.
• Beat egg white and 1 tablespoon water; brush over bottom of crust and set aside.
• Combine 3/4 cup sugar, cornstarch, lemon peel, vanilla extract, and cinnamon in a bowl. Gently stir blueberries and raspberries into the mixture to coat; pour into the crust. Top with reserved pie crust. Brush top of pie crust with 1 tablespoon water; sprinkle 1 tablespoon sugar over the moistened top. Cut a few slits into top crust with a sharp knife.
• Bake in preheated oven until crust is golden brown and filling is bubbly, about 45 minutes. Set pie onto wire rack to cool completely before refrigerating.