In a bowl, combine flour and cold butter. Use your fingers or a pastry cutter to mix until it forms coarse crumbs.
Add 2 tablespoons of ice cold water and stir until the dough comes together. Shape it into a disc and refrigerate for 30 minutes.
Toss the blueberries with cornstarch. Add lemon juice and sugar, mixing well.
Preheat your oven to 375F.
Roll the dough into a 12-inch circle and place it on a lined baking sheet. Put the blueberries in the center, leaving a 2-inch border around the edges. Fold the dough over the blueberries.
Whisk the egg with the remaining tablespoon of water and brush the edges of the dough. Sprinkle some extra sugar on top and bake for 30-35 minutes.