In a large bowl, toss together plums, 1 cup (200 grams) granulated sugar, lemon zest and juice, fresh ginger, ground ginger, salt, and pepper. Let stand for 30 minutes.
On a lightly floured surface, roll half of dough into a 12-inch circle. Transfer to a 9-inch pie plate, pressing into bottom and up sides. Trim dough to ½-inch beyond plate. Fold edges under and crimp as desired. Freeze for 30 minutes.
Preheat oven to 400°F.
Strain plum mixture through a colander into a large bowl. Reserve ½ cup plum liquid; discard remainder.
In a small saucepan, combine reserved ½ cup plum liquid, pectin, and remaining 1 tablespoon (12 grams) granulated sugar. Cook over medium-high heat, whisking constantly, until pectin dissolves and mixture thickens slightly, 3 to 4 minutes.
Toss drained plum mixture with cornstarch. Pour into prepared crust. Drizzle with pectin mixture.
On a lightly floured surface, roll remaining dough into a 12-inch circle. Cut dough into triangles, and layer on top of filling. Brush dough with beaten egg, and sprinkle with turbinado sugar. Place pie on a baking sheet.
Bake for 20 minutes. Reduce oven temperature to 375°F, and bake until golden brown and bubbly, 35 to 40 minutes more. Let cool for at least 1 hour before slicing.
Recipe by Bake from Scratch at https://bakefromscratch.com/california-plum-pie/