Mix the flour and salt in the food processor. Add the very cold butter cubes and pulse until the butter pieces are about the size of peas. Add the lightly beaten egg and cold water and pulse just a few times. The dough should come together when you pinch it. Turn the dough onto the working surface and divide into two even parts. Form each part into a disc. Wrap in plastic wrap and refrigerate until you make the filling.

Mix together the sugar, and corn starch. Clean and chop the rhubarb into pieces about 2 cm/ 0.8 inches. Mix with the blueberries in a large bowl. Add the sugar mixture and mix well to coat.

Roll one disc of dough slightly larger than the pie dish, so that you will have some dough overhanging over the edges of the dish. Place it in the pie dish. Cover with the rhubarb-blueberry mixture.

Roll the second disc of dough, again slightly larger than the pie dish. Place on top of the pie dish and press the edges with your fingers to seal the pie. Cut excess dough, if necessary. Cut a cross in the middle of the pie, so that the hot air can escape during baking. Place the pie in the fridge for about 20 minutes.

In the meantime, preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Mix together the egg and 1 tablespoon water. Brush the crust with the mixture. Sprinkle with some granulated sugar.

Bake in the preheated oven for 55-60 minutes until golden brown. Place the dish on a wire rack to cool. Slice the pie in the dish and serve it directly from it.