Dough: In a large bowl, whisk together salt and flour. With a pastry cutter or by hand, combine flour and cubes of butter until butter crumbles are the size of peas. Add ricotta, lemon, and water to the flour mixture, then mix just until combined (don't over work the dough, it should hold together loosely). Shape into a ball, flatten slightly to form a thick disc, wrap with plastic wrap, and set in the fridge for at least 1 hour (or pop in the freezer for 15 minutes)
Filling: Meanwhile, gently combine blueberries, cornstarch, sugar, lemon, and salt.
Assemble: On a floured surface, roll dough into a 12 to 14 inch circle. Set on a parchment paper-lined baking sheet and spread on ricotta, leaving 2 inches free around the edges. Spoon blueberry mixture over the ricotta, gently pressing down to flatten the mixture. Fold free dough over the blueberries, brushing the dough with whisked egg for a shiny golden finish and sprinkling with a bit of sugar.
Bake: Bake at 400 degrees F (204 C) for 30 to 40 minutes, or until crust is golden brown.