In a food processor, combine flour, granulated sugar, and kosher salt. Add cold unsalted butter and pulse until you achieve a coarse texture.

Gradually drizzle in ice-cold water until the dough comes together. Shape into a disk, wrap in plastic, and refrigerate for one hour.

Slice plums and combine them with sugar. Let sit for about 15 minutes to release juices.

Mix in cinnamon and tapioca flour, discarding excess juice to avoid a soggy crust.

Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.

Roll out the chilled dough into an 11-12 inch circle and transfer to prepared baking sheet.

Spoon plum filling onto the center, leaving a 1-1.5 inch border. Fold edges over fruit.

Whisk together egg and milk for egg wash. Brush exposed crust with the mixture and sprinkle with almond and sugar.

Bake for 30-35 minutes until the crust is golden brown. Let cool on a wire rack for about 2 hours.