How to Make It
Preheat oven to 400°F. Roll 1 of the piecrusts into a 12-inch circle on a lightly floured surface. Fit into a lightly greased (with cooking spray) 9-inch pie plate; fold edges under. Line dough with parchment paper, and fill with pie weights or dried beans. Bake in preheated oven 15 minutes. Remove weights and parchment paper, and bake until crust is lightly browned, 4 to 5 more minutes. Transfer to a wire rack, and cool completely, about 30 minutes.
Meanwhile, process tapioca in a spice grinder or food processor until finely ground (almost powder consistency), about 1 minute. Stir together ground tapioca, brown sugar, flour, cinnamon, cardamom, nutmeg, and 1/2 cup of the granulated sugar in a small bowl. Stir together rhubarb and lemon zest in a large bowl; add to tapioca mixture, and stir until well combined. Let stand, stirring occasionally, until some of the juices from the rhubarb are released, about 30 minutes.
Roll remaining piecrust into a 10-inch circle on a lightly floured surface. Using a 1 1/4-inch round cookie cutter, cut out 5 to 7 circles (leaving at least 1 inch of space along edges), reserving the 10-inch circle with cutouts.
Evenly sprinkle remaining 2 tablespoons granulated sugar on cooled piecrust; spoon rhubarb mixture evenly over sugared crust. Brush edges of crust with some of the beaten egg. Place the 10-inch circle over pie, and press edges of dough to baked crust to seal in filling. Brush 1 side of the cutouts with some of the beaten egg. Arrange cutout circles in a design by placing, egg side down, on top of pie.
Place pie on a rimmed baking sheet, and bake in preheated oven 15 minutes. Remove from oven, and cover edges with aluminum foil; bake until top is golden brown and pie is bubbling, 40 to 45 minutes. Transfer to a wire rack, and cool at least 2 hours before serving.