How to Make It
Line a baking sheet with parchment. Whisk together the sugar, cornstarch, tapioca, and salt in a medium bowl until the tapioca is broken up. Add the rhubarb and ginger and toss to combine.
Roll the piecrust on a floured surface into an 11-inch circle. Place on the prepared baking sheet. Spoon the fruit in the center of the piecrust, leaving a clear 2-inch border. Fold the edges of the piecrust over toward the center, overlapping slightly and partially covering the fruit. Chill until the dough is firm, at least 20 minutes and up to 1 hour.
Heat oven to 350° F. Brush the piecrust with the cream. Bake on the bottom rack until the juices are bubbling and the crust is golden, 50 to 60 minutes. Serve warm or at room temperature.