How to Make It
Combine flour, butter, oats, brown sugar, and 1 teaspoon salt in a medium bowl. Pinch and smear butter between your fingers until mixture is sandy and butter is evenly incorporated. Chill until ready to use.
Preheat oven to 375°F with oven rack in lower third of oven. Unwrap chilled dough disk, and place on a lightly floured work surface. (If dough has been chilled more than 8 hours, let stand at room temperature 10 to 15 minutes.) Roll into a 12-inch round. Transfer to a 9-inch pie pan with a lip; use your fingertips to relax dough into shape of pan. Trim dough, leaving a 1-inch overhang around edge of dish. Fold edge of dough under itself. Use your thumb and forefinger to crimp edge of dough, pressing crimp into pan and ensuring dough extends to outer edge of dish. (This anchors the pie crust, which results in a proud crown of crust after baking.) Freeze crimped dough, uncovered, until very cold, about 20 minutes.
Line crust with heavy-duty aluminum foil, leaving 1 1/2 inches of overhang. Trace fingertips over foil overhang, gently pressing into crimp. Fill foil-lined crust with pie weights or dried beans. Bake in preheated oven until crust is light golden around edges, 20 to 25 minutes. Remove pie weights and foil; prick bottom of crust all over with a fork. Return to oven, and bake at 375°F until bottom of crust is lightly browned, 6 to 8 minutes. Transfer to a wire rack to cool completely, about 30 minutes. While crust cools, reduce oven temperature to 350°F.
Toss together berries, granulated sugar, cornstarch, lemon juice, and remaining 1/4 teaspoon salt in a large bowl until berries are coated. Squeeze about 1 handful of berries, breaking them open. Stir in juices and smashed fruit. (This helps to create a sauce by introducing a small amount of berry juices to filling before baking.)
Spoon berry filling into cooled crust. Sprinkle with chilled crumb topping, sprinkling all the way to edge of crust. Place pie in oven. (If desired, slide an aluminum foil–lined baking sheet onto oven rack below pie to catch any drips.) Bake at 350°F until filling is bubbling, about 1 hour and 30 minutes, shielding edges with foil after 25 minutes to prevent excess browning, if necessary. Transfer pie to a wire rack, and let cool completely, about 3 hours. Serve pie slices with cold heavy cream for drizzling.