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.
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.
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.
Filling can be made in advance; cover tightly and keep chilled. If needed, warm it up slightly to easily fill pie shell. For our full blueberry pie recipe including all-butter crust, see here. If you enjoyed this recipe, please come back and give it a rating ♡