Stir the flour and salt together in a large bowl. Cut in the cold butter and shortening until the mixture resembles coarse oatmeal with some pea-sized bits left.

Beat the egg in a measuring cup. Add vinegar and top up with ice-cold water to the 1-cup mark. Stir this into the flour just until the dough holds together.

Divide the dough into 5 portions, flatten into discs, and wrap tightly. Refrigerate 2 discs for 1 hour, and freeze the other 3.

Preheat the oven to 350°F. Roll one chilled disc into a 12-inch circle and fit it into a 9-inch pie plate. Toss the berries with sugar and cornstarch, spread them in the crust, and dot with butter.

Roll the second disc into a 12-inch circle, cut into strips, and weave into a lattice. Seal edges and brush the top with egg wash, sprinkling sugar on top.

Bake for 90-100 minutes until the crust is golden and the filling bubbles. Cool on a wire rack and serve warm or at room temperature.