Step 1

Cook: Place 3 cups of berries in a saucepan and set over medium heat. Mash berries several times to release juices. Continue cooking, stirring often and occasionally mashing, until half of the berries have broken down. Mixture should be thickened and reduced to 1 1/2 cups, about 8 min. Let cool slightly.

Step 2

Combine: Place grated apple in a clean cloth and wring to dry. Transfer apples to large bowl and add cooked berries, the remaining uncooked berries, lemon zest, lemon juice, sugar, cornstarch, cinnamon, and salt. Toss to combine.

Step 3

Fill: When ready to use, use rubber spatula to transfer filling into pie shell, scattering butter pieces on top of filling, prior to sealing with upper pie dough.