Preheat an oven to 220 degrees C. Line a baking tray with foil.
Toss the rhubarb and blueberries in a bowl with the salt, nutmeg, lemon juice, 1 1/3 cup sugar and flour until evenly mixed. Pour into the pie crust and dot with butter. Cover the filled crust with the top pastry and flute the edges. Cut a few decorative steam vents in the top pastry.
Sprinkle the pie with the caster sugar then cover the fluted edges with foil to prevent excessive browning. Place the pie on the prepared baking tray.
Bake in the preheated oven for 15 minutes then reduce the heat to 180 degrees C and bake 35 minutes longer.
Remove the foil and continue baking until the pastry is golden and the juice is bubbling through the slits; about 15 minutes more. Cool completely before serving.