Instructions

for the crust

  Shred 4 tablespoons butter on large holes of box grater and place in freezer. Cut remaining 16 tablespoons butter into ½-inch cubes.
 Pulse 1½ cups flour and salt in food processor until combined, 2 pulses. Add cubed butter and process until homogeneous paste forms, 40 to 50 seconds. Using your hands, carefully break paste into 2-inch chunks and redistribute evenly around processor blade. Add remaining 1 cup flour and pulse until mixture is broken into pieces no larger than 1 inch (most pieces will be much smaller), 4 to 5 pulses. Transfer mixture to medium bowl. Add shredded butter and toss until butter pieces are separated and coated with flour.
 Sprinkle ¼ cup ice water over mixture. Toss with rubber spatula until mixture is evenly moistened. Sprinkle remaining ¼ cup ice water over mixture and toss to combine. Press dough with spatula until dough sticks together. Use spatula to divide dough into 2 portions. Transfer each portion to sheet of plastic wrap. Working with 1 portion at a time, draw edges of plastic over dough and press firmly on sides and top to form compact, fissure-free mass; wrap in plastic and form into 5-inch disk. Refrigerate dough for at least 2 hours or up to 2 days. Let chilled dough sit on counter until softened slightly, about 10 minutes, before rolling. (Wrapped dough can be frozen for up to 1 month. If frozen, let dough thaw completely on counter before rolling.)

for the filling

  One hour before baking pie, make filling. Toss onion and salt in bowl and set aside. Bring 4 quarts water to boil in Dutch oven over high heat. Add potatoes and baking soda. Return to boil and cook for 1 minute. Drain potatoes. Return potatoes to pot; add cream, garlic, pepper, nutmeg, and onion and any accumulated liquid; and bring to simmer over high heat. Adjust heat to maintain simmer and cook, stirring gently and frequently (it's OK if some slices break), until cream thickens and begins to coat potatoes, about 5 minutes. Let cool off heat for at least 30 minutes or up to 2 hours.
 Roll 1 disk of dough into 12-inch round on well-floured counter. Loosely roll dough around rolling pin and gently unroll onto 9-inch pie plate, letting excess dough hang over edge. Ease dough into plate by gently lifting edge of dough with your hand while pressing into plate bottom with your other hand. Refrigerate until dough is firm, about 30 minutes. Roll second disk of dough into 12-inch round on well-floured counter, then transfer to parchment paper–lined baking sheet; refrigerate for 30 minutes. Adjust oven rack to lower-middle position and heat oven to 450 degrees.
 Stir parsley into potato mixture, transfer mixture to dough-lined pie plate, and spread into even layer (it's OK if potato mixture is still slightly warm). Using paring knife or round cutter, cut ½-inch hole in center of second dough round. Loosely roll dough round around rolling pin and gently unroll it over filling, aligning hole with center of pie and leaving at least ½-inch overhang all around. Fold dough under itself so edge of fold is flush with outer rim of pie plate. Flute edges using your thumb and forefinger or press with tines of fork to seal. Place pie on parchment-lined rimmed baking sheet and brush with egg. Bake until top is light golden brown, 18 to 20 minutes.
 Reduce oven temperature to 325 degrees and continue to bake until crust is deep golden brown and potatoes at vent hole are tender when pricked with paring knife, 30 to 40 minutes longer. If pie begins to get too brown before potatoes are softened, cover loosely with aluminum foil. Let pie cool on wire rack for at least 30 minutes. Serve warm or at room temperature.