Preheat the oven to 400°F. Line two large baking sheets with parchment paper.
Toss together the rhubarb, blueberries, sugar, flour, lemon zest, salt, cinnamon, and cardamom in a large bowl to combine.
Place one sheet of puff pastry on a lightly floured work surface and roll out to a 12-inch square. Cut into 4 smaller squares. Distribute some of the rhubarb mixture in the center of each square of dough and fold the pastry over the fruit to form a triangle. Seal the edges with the tines of a fork. Brush the tops with the egg wash and cut a slit in the top of each turnover. Place on the prepared baking sheets. Repeat with the remaining puff pastry and rhubarb filling.
Bake for 20 to 22 minutes, until golden brown all over. Remove from the oven and let cool slightly before serving.