In a large bowl, mix the flour and butter until the mixture looks like coarse crumbs. Add two tablespoons of water and blend until a thick dough forms. Shape the dough into a disc and refrigerate for 10 minutes.

In another bowl, combine the berries with the cornstarch, then mix in the sugar and lemon juice.

Preheat your oven to 375F and line a baking sheet with parchment paper.

Roll out the dough into a 12-inch circle on the lined baking sheet. Spoon the berry mixture into the center, leaving about two inches of dough around the edges. Fold the edges over the fruit.

Whisk the egg with the remaining tablespoon of water and brush this around the edges of the dough. Sprinkle the dough with superfine sugar.

Bake for 30-35 minutes until the crust is golden brown. Let the crostata cool for 5 minutes before slicing.