Preheat oven to 400 degrees F.

Combine the chopped rhubarb, blueberries, granulated sugar, corn starch, and cardamom in a mixing bowl and stir to combine.

Roll out your pie dough or spread out your prepared pie dough and place it on a parchment-lined baking sheet.

Top the pie crust with the rhubarb mixture, leaving a 1 ½ inch border.

Fold the edges of the pie crust up over the filling in pleats. Brush the edges with the beaten egg. Sprinkle with a pinch of cinnamon sugar or demerara sugar.

Bake for 40 minutes or until the dough is golden brown.

Allow to cool for 10 minutes before serving.