How to Make It
Preheat oven to 350°.
Pulse together flour, butter, and 1 teaspoon salt in a food processor until a coarse meal forms. Pulse in water 1 tablespoon at a time, just until dough can hold together when lightly squeezed. Divide dough in half. Press each into a (6-inch) circle between 2 sheets of plastic wrap. Wrap tightly and refrigerate 30 minutes.
Combine sugar, remaining 2 tablespoons flour, zest, and remaining ½ teaspoon salt in a large bowl. Rub together with your fingers to evenly disperse oil from zest. Add blueberries and vanilla; stir to combine. Place blueberry mixture in an 11-x7-inch, 2-quart baking dish (it will mound over the top).
Working one disk at a time roll dough on a floured work surface to 1/8-inch thickness. Cut into 1-inch strips. Top the blueberry mixture in baking dish with dough strips to form a “lattice” pattern, leaving 1/2-inch squares between the lattice and pressing dough firmly to the sides of the dish so it adheres. Brush top with egg whites and sprinkle with Demerara sugar. Bake at 350° for 50 minutes to 1 hour or until bubbly and top is golden, tenting with foil after 25 minutes if the crust begins to brown too quickly. Let stand 10 minutes before serving. Serve warm with vanilla ice cream, if desired.