7 cups frozen mixed berries (blackberries, blueberries, and raspberries)
¾ cup granulated sugar
⅓ cup arrowroot powder
½ teaspoon salt
Preheat the oven to 375 degrees F (190 degrees C).
Lightly dust an 11×15-inch half-sheet pan with flour. Carefully place the 2 pie crusts into the bottom of the sheet pan, gently stretching and trimming the crusts to fit the bottom and up the sides of the pan.
Combine berries, sugar, arrowroot powder, and salt in a large bowl; stir well.
Combine oats, flour, brown sugar, apple pie spice, and salt in a medium bowl. Use a pastry knife to cut butter into the oat mixture until butter is evenly distributed throughout and the crumble topping mixture resembles small, uneven pieces.
Pour berry mixture evenly onto the crust and add crumble mixture on top.
Bake in the preheated oven until brown and bubbly, 45 to 55 minutes. Remove and allow to cool for 30 to 45 minutes