Add the flour and salt to the bowl of a food processor and pulse several times to combine.

Cut the butter into small pieces, or grate on a box-grater, add to the flour and pulse for 30 seconds or until only small pea-size bits of butter remain. Sprinkle the mixture with 5 or 6 tablespoons of ice water and pulse the processor for about 30 seconds or until the flour mixture comes together in a mass. Do not over-process, any remaining crumbs in the processor bowl can be worked into the dough on the work surface.

Transfer the dough to a lightly dusted work surface.  Shape the dough (incorporating any of the remaining flour mixture crumbs) into a ball. 

Divide the pastry ball into two sections, one a little larger than the other and flatten each into a disk. Wrap each ball dough in plastic wrap and refrigerate for 30 minutes. 

Remove the chilled pastry from the refrigerator and roll the larger disk into a large circle about 12-inches in diameter. Use a bench scraper or firm flat spatula to turn the dough and work flour underneath as you roll it. Transfer the pastry to a 9-inch deep-dish pie plate and reserve.

Preheat the oven to 400°F and arrange the oven rack to the lowest position.

Measure the rhubarb and blueberries into a large bowl.  Drizzle the lemon juice over the top of the berries and toss together to combine.  

In a small bowl combine the sugar, salt, tapioca, cinnamon and nutmeg, add the mixture to the fruit and toss until well combined.

Spoon 1/2 of the fruit mixture into the prepared pie crust and dot with 1/2 of the butter pieces.  Spoon the remaining fruit mixture into the crust and add the remaining butter pieces on top. 

Roll out the top crust and place over the fruit mixture, trim the excess dough and crimp the edges.  

Brush the egg/water wash lightly over the top of the pie, cut steam vents and place the pie on a parchment-lined baking sheet.  (This is a safety net and will save you a clean-up if the pie drips.)

Transfer the baking sheet/pie to the oven and bake for 25 minutes until light golden brown.

Reduce the oven temperature to 350°F and bake for 30 minutes.  

Open the oven door and sprinkle the top of the pie with the coarse sugar.  Continue baking for an additional 25-30 minutes or until juices are bubbling from the pie vents and the top is a deep golden brown. (See notes)

Remove the pie from the oven and transfer to a wire rack for 2 hours, then refrigerate until cold before serving.