In a large bowl, whisk together flour, salt, and sugar. Cut in cold butter until the mixture resembles coarse crumbs.

Add ice water one tablespoon at a time, mixing until the dough just comes together.

Divide dough into two disks, wrap in plastic wrap, and refrigerate for at least 1 hour.

Preheat oven to 375°F (190°C).

In a large mixing bowl, combine blueberries, sugar, cornstarch, lemon juice and zest, vanilla extract, lavender, and salt. Stir gently and let sit for 10 minutes.

Roll out one dough disk and press into a 9-inch pie dish. Pour in the blueberry filling.

Roll out the second disk and cut into strips. Create a lattice over the top of the pie.

Brush the lattice crust with egg wash and sprinkle with coarse sugar if using.

Bake for 45–55 minutes, or until the crust is golden and the filling is bubbly. Cover the edges with foil if browning too quickly.

Let the pie cool completely before slicing to allow the filling to set.