Preheat your oven to 400°F (204°C) and line a baking sheet with parchment paper.

In a large mixing bowl, combine rhubarb, blueberries, sugar, corn starch, and optional cardamom, stirring gently for 2-3 minutes.

Roll out the pie dough into a 12-inch circle and transfer it to the prepared baking sheet.

Spread the rhubarb and blueberry mixture evenly on the dough, leaving a 1½ inch border.

Fold the edges of the pie crust over the filling and brush with beaten egg. Sprinkle with sugar if desired.

Bake for 40 minutes, or until the crust is golden brown and the filling is bubbling.

Cool on the baking sheet for about 10 minutes before serving.