In a food processor, combine the flour, almond flour, sugar, and butter.
Add the sour cream, water, and almond extract while the machine is running.
Process until the dough forms a ball.
Remove the dough from the processor, flatten it, and wrap it in plastic wrap. Refrigerate for 30 minutes.

Meanwhile, preheat the oven to 400 F (200 C).
Line a baking sheet with parchment paper. Set aside.

On a lightly floured surface, roll out the dough into a 1/4″ (0.5 cm) thick circle.
Gently place the dough on the prepared baking sheet and set aside.

Cut the plums into 6 or 8 pieces, remove the pits, and place them in a bowl.
Add the flour, sugar, and melted butter, and gently mix with your fingers until all the plum slices are well coated.

Place the filling in the center of the reserved dough, leaving a 1″ (2.5 cm) border.
Fold the border over the fruit, allowing the dough to pleat.
Brush the folded dough with the beaten egg.
Sprinkle with sliced almonds.
Bake for 40 to 45 minutes.
Remove from the oven and let cool on the baking sheet, allowing the juices to set.