Start by grating frozen unsalted butter into a mixing bowl filled with all-purpose flour, granulated sugar, and fine sea salt. Blend until it resembles pebble-sized crumbs.

Gradually mix in ice water until the dough just starts to clump together. Be careful not to overwork it.

Divide the dough into two portions, wrap each in plastic, and refrigerate for at least 30 minutes.

Combine frozen blueberries, brown sugar, and lemon juice in a saucepan over medium heat. Cook until juices are released.

Mix cornstarch with water, add to the blueberry mixture, and cook until thickened.

Preheat oven to 375°F. Roll out one portion of chilled dough, place in greased baking dish, prick with a fork, and bake until golden.

Roll out the second dough portion and cut into strips. Weave into a lattice pattern and freeze for 10 minutes.

Spread blueberry filling over the baked crust, place the lattice on top, trim excess dough, crimp edges, brush with egg, and sprinkle with sugar.

Return to oven and bake until the lattice is golden brown and filling is bubbling. Allow to cool completely before slicing.