Creating a Blueberry Lemon Galette is straightforward if you follow these step-by-step instructions:

1. Make the Dough: In a large bowl, combine the flour, granulated sugar, and salt. Add the cold, cubed butter and mix until the mixture resembles coarse crumbs.
2. Add Water: Gradually add the ice water, one tablespoon at a time, mixing until the dough holds together. Do not overwork it.
3. Chill the Dough: Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
4. Prepare the Filling: In a medium bowl, mix the blueberries, lemon juice, lemon zest, cornstarch, and 2 tablespoons of sugar until well combined. Set aside to let the flavors meld.
5. Preheat the Oven: Preheat your oven to 400°F (200°C).
6. Roll Out the Dough: On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer it to a parchment-lined baking sheet.
7. Add the Filling: Spoon the blueberry mixture into the center of the dough, leaving a 2-inch border around the edges.
8. Fold the Edges: Gently fold the edges of the dough over the filling, crimping it slightly to hold its shape.
9. Egg Wash: Beat the egg and brush it over the exposed dough. Sprinkle coarse sugar on top for added sweetness and crunch.
10. Bake: Place the galette in the preheated oven and bake for 40-45 minutes, or until the crust is golden brown and the filling is bubbling.
11. Cool: Remove from the oven and allow the galette to cool for about 15 minutes before slicing.

By following these steps, you will create a stunning Blueberry Lemon Galette that will delight everyone at your table!