If using whole freeze-dried blueberries (not powder), place them into a small food processor or pestle and mortar then grind until the blueberries have been broken down into a fine powder.
Take 2 tablespoons of this homemade blueberry powder (or, if you bought it, the store-bought blueberry powder) and combine with the water in a small bowl.
Place the flour, salt and sugar into a food processor along with the cold butter cubes. Pulse until the butter has been broken down so the mixture looks crumbly but there are some pea-sized butter lumps remaining. Pour in the blueberry water mixture and pulse again until just combined – you may need an extra tablespoon of water if the dough is looking too dry.
Tip the dough out and use your hands to bring it together into a ball. Divide this in half and form two disks – you want one to be slightly larger as the excess will be needed for making the decorations for the crust.
Wrap the dough disks in plastic wrap or place into re-sealable plastic bags and chill for at least 1 hour.
To make the filling:
Place half of the blueberries, all of the sugar, the lemon zest and lemon juice into a medium pot. Set over a medium heat and bring to a boil then turn down to simmer.
Stir the cornstarch, cinnamon, nutmeg and water together in a small bowl until smooth. Pour this into the pot with the blueberries and stir until thickened. Remove from the heat and stir in the remaining blueberries. Set aside to cool.
Take the smaller, chilled disk of pastry and dust lightly with flour. Roll out on a large piece of parchment paper into a circle slightly larger than your 9-inch pie pan. Flip the dough and parchment together onto the pie pan. Peel away the parchment and press the dough down gently so it lines the pan. Pour in the chilled filling and spread it out into an even layer.
Roll the second, larger, chilled disk of dough out on the same piece of parchment as you were using earlier, dusting with flour as needed. You want this circle of dough to be a couple of inches larger than the pie tin. Use a 1 inch round pastry cutter to randomly cut holes all over the circle of dough. Reserve all of the 1-inch dough circles this creates as you’ll need them for decoration.
Carefully flip the dough and parchment together over the filled pie dish. Peel away the parchment paper gently so you don’t tear any of the circles. Press the dough together all around the edge of the pie pan and then use a sharp knife to trim away the excess pastry.
Re-roll this pastry to about 1/8 to 1/4 -inch thick. Use the same 1-inch round cutter to cut out more circles from the dough. In total, you want 30-40 of these small circles.
Crack the egg into a bowl and beat lightly with a fork. Brush this around the edge of the pie ONLY. Take the 1-inch circles of dough and place them around the edge of the pie, letting them overlap slightly.
Chill the pie for 15 minutes as the oven preheats to 350 degrees F.
Bake the pie for 40 minutes until the crust is starting to brown and the filling is bubbling.