In a mixing bowl, combine the flour, sugar, and salt.

Add the cold butter and rub it into the flour with your fingertips until the mixture resembles coarse crumbs.

Add the egg and mix it in. If needed, add cold water, one tablespoon at a time, until the dough just comes together. Do not overwork the dough to maintain a flaky texture.

Shape the dough into a disk, wrap it in plastic wrap or a reusable wrap, and refrigerate for at least 30 minutes.

Lightly flour a work surface and roll out the chilled dough to fit a 9-inch (23 cm) pie dish, about ¼ inch (5 mm) thick.

Carefully transfer the rolled-out dough into the pie dish, gently pressing it into the base and sides. Prick the bottom with a fork to prevent bubbling.

In a saucepan, whisk together the sugar, cornstarch (cornflour), vanilla sugar (or extract), and salt.

Add the slightly thawed frozen berries and lemon juice. Stir well to coat the berries evenly.

Cook over medium heat, stirring continuously, until the mixture thickens and becomes glossy (about 5–7 minutes).

Take the filling off the heat and use immediately while warm.

Set the oven to 375°F (190°C).

Pour the hot berry filling into the prepared pie crust and spread it evenly.

Place the pie on the lower oven rack and bake for 35–40 minutes, or until the crust is golden brown and the filling is bubbling and slightly set.

Allow the pie to cool for at least 1 hour before slicing. This helps the filling firm up and prevents excess liquid from seeping out.