• In a large saucepan, combine the frozen blueberries, cornstarch, lemon zest, lemon juice, cinnamon, sugar, and salt. Don't add the butter yet. On very low heat, stir until the frozen blueberries release some of their liquid.
• Once they've released their juices, turn the heat up to medium and keep stirring regularly until the mixture heats thoroughly and comes to a bubble. Don't forget to stir, or the cornstarch mixture will begin to cook prematurely at the bottom of the pan.
• Once the mixture begins to bubble, let it boil while continuously stirring. Do this for a full minute until it's thick and glossy. It will lose its cloudy appearance and become clear and thick.
• Once it's cooked, remove it from the heat and stir through the tablespoon of butter if using it.
• Pour the hot blueberry mixture into a large bowl and let it cool to room temperature. The filling can also be made a couple of days in advance and stored in the fridge in an airtight container.

Assembling

• Roll half the pie dough on a floured work surface into a 12-inch circle. Drape it over a pie dish or tart pan, leaving an overhang. Scoop in the cooled blueberry filling.
• Cover the top of the pie with the second half of the pie dough. You can simply roll another 12-inch circle, drape it on top, and flute the edges. Make 2-3 slits in the middle to allow steam to escape. Alternatively, take your time and make a lattice crust. See the notes for how to make a lattice top.
• Cut off the excess dough but leave an overhang. Roll the top and bottom crusts together. You can flute the edges or use a fork to create tine patterns.
• If you're using a tart pan intead of a pie dish, then, instead of fluting the edges, you can press these down so the edges of the tart pan cut all the excess dough off. (See the above post for a visual)
• Mix together the egg and water. Using a pastry brush, brush the dough on top of the pie with egg wash.

Baking

• Place the unbaked pie in the fridge for at least 30 minutes and preheat the oven to 425°F (220°C.)
• Once the oven is at temperature, place the pie on the middle rack in the oven, and add a tray underneath to catch any spills.
• Bake at 425°F (220°C) for 15 minutes, then turn the heat down to 375°F (190°C) and continue baking for a further 45 minutes until the pie crust is thoroughly browned and the purple blueberry filling bubbles through the gaps in the top crust.
• If the crust is showing signs of excessive browning, you can cover the top of the pie with a piece of aluminum foil. However, don't be afraid of a deep golden brown crust, as this brings flavor and flakiness. 
• Once baked, remove the hot pie from the oven and let it cool on a wire rack for at least 4 hours. For a firmer filling, let the pie cool completely and refrigerate it for a few hours once it is chilled.
• Serve with a scoop of vanilla ice cream.