In a large mixing bowl, combine flour, sugar, and salt. Transfer to food processor, add butter, and pulse until mixture resembles coarse peas. Gradually add cold water while pulsing until dough holds together. Shape into a disk, wrap, and refrigerate for 1 hour.

Preheat oven to 400°F (200°C). Slice plums into wedges and toss with 2 tablespoons of sugar. Let sit for 5-10 minutes to release juice.

Roll the chilled dough into an 11-12 inch circle on a floured surface. Transfer to parchment-lined baking sheet. Brush with egg wash, leaving a border.

After draining excess juice, combine plums with remaining sugar, salt, tapioca flour, and cinnamon. Spread filling in the center of dough, fold edges over fruit, and brush with egg wash, adding optional toppings.

Bake for 30-35 minutes until edges are golden and plums bubble. Cool on wire rack for 2 hours before serving.