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

In a large mixing bowl, combine chopped rhubarb, blueberries, granulated sugar, corn starch, and cardamom or cinnamon. Stir until well-coated.

Roll out the pie crust until it reaches about 12 inches in diameter. Transfer it to the parchment-lined baking sheet.

Spoon the fruit filling into the center of the crust, leaving a 1½-inch border around the edges.

Fold the crust edges over the filling, brush with beaten egg, and sprinkle with cinnamon or demerara sugar if desired.

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

Let the galette cool on the baking sheet for about 10 minutes before serving.