If your food processor is not large enough to hold the entire amount of dough, divide the ingredients in half and make the recipe in two batches, then combine the doughs before dividing into thirds.
Crust: Pulse flour and salt in a food processor to combine. Add butter; pulse until mixture resembles coarse crumbs with some larger pieces remaining. Add ice water; pulse until dough just starts to come together. Divide dough into thirds, pat into disks, and wrap each in plastic; refrigerate at least 1 hour or overnight.
Let dough stand at room temperature until pliable. Roll out 1 disk on a lightly floured surface to an 11-inch round, about 1/4 inch thick. Fit into a 9-inch pie plate. Trim overhang to just beyond rim of pie plate; tuck under to create a neat edge all around. Refrigerate until firm, about 30 minutes.
Meanwhile, roll out remaining 2 disks of dough to just under 1/4 inch thick; freeze on 2 baking sheets until firm, about 30 minutes. Using a 1-inch fluted cookie cutter, cut out as many rounds as possible from sheets of dough (rerolling and cutting out scraps once). Return cutouts to freezer.
Filling: Toss blackberries with lemon juice, granulated sugar, oregano, cornstarch, and salt in a bowl. Transfer to piecrust. Brush edge of piecrust with beaten egg and arrange overlapping rounds of dough on top of edge. Continue overlapping rounds to cover entire top. Freeze 1 hour.
Preheat oven to 400 degrees with a rack in lower third and a baking sheet on rack below. Brush entire top of pie with egg wash and sprinkle with sanding sugar. Bake 30 minutes. Reduce heat to 375 degrees and continue to bake until juices are bubbling near center (if browning too quickly, tent top or edge with foil), about 1 hour more. Let cool completely, at least 6 hours or overnight. Serve with whipped cream.
