How to Make It
Prepare the Crust: Pulse flours and salt in a food processor until combined. Add butter, and pulse until loose crumbs form, 4 to 5 times. Add egg; pulse until dough comes together, about 3 times. Place dough on a sheet of parchment paper, and press into a 6-inch disk. Wrap with parchment, and refrigerate at least 1 hour or up to overnight.
Preheat oven to 375°F. Dust a 15-inch square sheet of parchment paper with tapioca flour. Unwrap dough disk, and place in center of dusted parchment sheet. Roll to a 12-inch circle. Invert parchment, fitting crust into a 9-inch round tart pan with removable bottom. Trim any dough overhang, and use to patch cracks or to build up sides, as needed. Refrigerate 30 minutes. Prick bottom all over with a fork. Top with a parchment sheet, and fill with pie weights. Bake in preheated oven 20 minutes. Remove pie weights and parchment. Continue to bake until crust is golden and dried, about 10 minutes. Shield edges with aluminum foil to prevent overbrowning, if necessary. Let cool completely on a wire rack, about 30 minutes.
Prepare the Filling: Heat juice, sugar, tapioca flour, and salt in a heavy saucepan over medium-high, stirring often, until small bubbles form around edges of pan, 6 to 8 minutes. Meanwhile, whisk yolks in a medium bowl. Add 1 cup hot juice mixture to the yolks, 1/4 cup at a time, whisking constantly until smooth. Add yolk mixture to pan with remaining juice mixture, and cook over medium heat, stirring often, until thickened and large bubbles form, 3 to 4 minutes. Continue cooking, stirring constantly, 1 minute. Remove from heat. Let cool slightly, about 2 minutes. Whisk in butter until smooth. Stir in food coloring. Let cool 15 minutes. Pour into unmolded tart shell, and refrigerate until set, 4 to 6 hours. Top with whipped cream, and serve.