Preparation

Make the streusel

• In a medium bowl, combine the flour, oats, brown sugar, and salt. Toss in the cold butter, and mix with your hands, rubbing the butter into the dry ingredients until moist crumbs form, about 1 minute.

Make the pie

• Position a rack in the bottom third of the oven and heat the oven to 425°F.
• Put the berries in a large bowl. In a small bowl, whisk the granulated sugar and cornstarch. Add the sugar mixture, zest, and juice to the fruit, and toss to combine.
• Spoon the fruit into the crust. Top with the streusel in an even layer. Transfer the pie to the oven. Bake until the crust is golden brown and the filling is bubbly, about 45 minutes. Rotate the pie halfway through baking for even browning; tent with foil if the pie is browning too quickly.
• Cool for at least 15 minutes before serving.