In a large mixing bowl, combine the flour, sugar, and salt for the crust. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.

In a small bowl, whisk together the egg and cold water. Gradually add the egg mixture to the flour mixture, stirring with a fork until the dough comes together.

Turn the dough out onto a lightly floured surface and knead it gently a few times until it becomes smooth. Shape it into a disk, cover it with plastic wrap, and refrigerate for 30 minutes.

Meanwhile, prepare the filling. In a saucepan, combine the orange juice, orange zest, and sugar. Bring the mixture to a simmer over medium heat, stirring occasionally, until the sugar has dissolved.

In a separate bowl, whisk the eggs. Slowly pour the hot orange juice mixture into the beaten eggs, whisking constantly to prevent the eggs from curdling. Stir in the orange liqueur if desired.

Take the dough out of the refrigerator and roll it out on a floured surface to fit a 9-inch (23 cm) tart pan. Carefully transfer the dough to the pan, pressing it gently into the bottom and sides.

Pour the orange filling into the prepared tart shell, spreading it evenly.

Place the tart in the preheated oven and bake for 35-40 minutes, or until the filling is set and the crust is golden brown.

Remove the crostata from the oven and allow it to cool completely before serving. Optionally, you can garnish with whipped cream or a dusting of powdered sugar.