Bit of advice: This will take time! It is not difficult, but it needs time to set up. Make this the morning of or day before you want to serve it. Trust me. Happy baking!
FILLING
STEP 1
Crust: Place flour, salt, and butter into a large mixing bowl. Cut with a pastry cutter until mixture resembles coarse crumbs about the size of peas. Add ice water stir 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. This is a very important step! If you do not do this the butter will melt while you are rolling it out.
STEP 2
Let dough stand at room temperature until pliable. Roll out 1 disk on a lightly floured piece of waxed paper to an 11-inch round, about 1/4 inch thick. Flip your waxed paper into a 9-inch pie plate. Pull away waxed paper and work the dough carefully into the corners. Trim overhang to just beyond rim of pie plate; tuck under to create a neat edge all around. Place in the refrigerater.
STEP 3
Meanwhile, roll out remaining 2 disks of dough on waxed paper to just under 1/4 inch thick; freeze on 2 baking sheets until firm, about 30 minutes. It may not take this long though. 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.
STEP 4
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. Taking one round of dough from the freezer carefully flip the round onto the pie filling. Press gently to seal the edges, and trip any extra dough. The dough will be stiff so let it settle on its own. Using a decorative cutter, cut out as many pieces as possible from the remaining sheet of dough from the freezer. Add beaten egg to pie crust and place cut-outs as desired. Place pie in freezer for one hour.
STEP 5
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 sugar in the raw. I love the beautifully sandy color, but if you prepare a cleaner look use 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.