- Place flour, sugar, and salt in bowl of food processor, and pulse to combine. Add cold butter and pulse until mixture resembles coarse cornmeal.
- With food processor running, slowly add ice water in thin, slow stream. Pulse again until dough just comes together.
- Take a bit more than one-third of the dough and pat into flat, round disc. Pat remaining dough into flat rectangle, and wrap both tightly with plastic wrap.
- Chill at least 4 hours, but preferably overnight.
- Roll disc of dough into 11-inch circle, about ⅛-inch thick, and place into 10-inch pie plate.
- Roll rectangle until ⅛-inch thick and a bit wider than pie plate on one side. Cut rectangle into 8 even strips using pastry cutter or sharp knife.
- On lightly floured parchment paper, alternate weaving strips to create lattice top. Place on sheet pan, and chill for 1 hour.
- Preheat oven to 400 degrees.
- Combine rhubarb and sugar in large bowl, and let sit 15 minutes to draw out juices. Add tapioca and salt, and stir to combine. Let sit additional 15 minutes to allow tapioca to hydrate.
- Position baking rack at lowest position in oven. Chef note: If your oven does not heat well from bottom, place flat cast-iron griddle pan or cast-iron pan wide enough to fit pie in, into cold oven. Preheat pan with oven. When oven comes up to temperature, carefully place prepared dough pie in cast-iron skillet or on griddle. Heat of cast-iron will ensure a nice, crispy bottom crust.
- Fill unbaked crust with rhubarb filling. Brush edges of crust with beaten egg. Carefully flip prepared lattice on top of pie. Use fork to press lattice onto edge of crust.
- Brush top with egg wash and sprinkle with coarse sugar.
- Place pie in oven, and reduce heat to 375 degrees. Bake until crust is deep, golden brown, and rhubarb juices are thick and bubbly, approximately 45 minutes.