In a bowl, mix rhubarb with granulated sugar and let sit until watery, stirring occasionally (will take at least 45 minutes - mine sat out for over an hour). Stir in blueberries, cornstarch, salt, cinnamon and nutmeg and mix well.

Preheat oven to 425°F.

Line a deep dish pie plate with one of the discs of dough, leaving an overhang. Pour blueberry-rhubarb mixture into the prepared pie plate and spread out evenly. Dot the top of the fruit with butter and top with the second pie crust (either rolled out solid or cut into ¾-inch wide strips to form a lattice). Crimp edges to join the top and bottom crusts. If using a solid top crust, cut a few steam vents in with a sharp knife.

Place pie plate on a baking sheet. Brush top of pie crust with a thin layer of the beaten egg, and sprinkle raw sugar over top. Cover edges of the crust with aluminum foil and bake for 15 minutes.

Lower the heat to 350°F and bake another 35 minutes. Remove foil from edge of crust and bake a final 15 minutes, or until the edges are brown and the fruit is thick and bubbly.

Remove from oven and cool pie completely on a cooling rack before slicing. Enjoy!