Whisk together the flour and salt in a large bowl. Add half the butter and cut in, using a round-bladed knife or pastry cutter. Combine well using a cutting motion. Add the remaining butter and continue to mix until you have roughly pea-sized pieces.

Sprinkle over the iced water (holding back the ice) and toss it through the mix as you go. The dough should start to become raggedy and eventually, when all the water is added, it will come together into a ball. Divide the ball in half, wrapping each piece in clingfilm. Pat them into flat squares and rest in the fridge for at least 20 minutes or up to 24 hours. (Any longer than this, put it in the freezer.)

To make the filling, strip the grapes off the stems, then separate the skins from the pulp by pinching each grape to squeeze out the pulp into a heavy-based saucepan. Set the skins aside. Add the scraped vanilla pod and seeds, plus the lemon zest and heat gently until the pulp is soft and beginning to separate from the grape seeds, about 10 minutes.

Pour the pulp through a mesh sieve placed over a bowl. Press through with a plastic pastry scraper or the back of a wooden spoon until all the pulp is in the bowl. Discard the grape seeds. Rinse and dry out the vanilla pod for another purpose. Add the lemon juice to the pulp.

In another large bowl, whisk together the sugars, cornflour, salt and nutmeg. Slowly whisk the strained grape mixture into the sugar mixture and add the grape skins back in. Add the apples to the grape mixture.

Butter and flour a baking tray that measures 23×33cm (9×13in) and has a lip about 2.5cm (1in) deep.

Roll out one square of pastry on a lightly floured surface to roughly 28×38cm (11×15in). Press the pastry down into the prepared baking tray, then chill in the fridge while you roll out the other piece. The second pastry sheet (which will form the top of the pie) can be rolled out to 23×33cm (9×13in). Remove the pastry-lined baking tray from the fridge and carefully fill it with the grape mixture. It can come right up to about 2mm shy of the top of the tin but don’t let it overflow. Roll the top layer of pastry over the pie. Brush the pastry with milk. Fold or roll over the excess pastry and pinch to seal. Use a knife to pierce the top of the pie a few times. Put in your freezer or fridge for 20 minutes.

Preheat the oven to 180°C fan/200°C/400°F/gas mark 6.

Brush the edge and top of the pie with the milk or cream and sprinkle over the caster sugar. Line the bottom of your oven with foil to catch any drips, then bake the pie for about 25 minutes. Reduce the heat to 160°C fan/180°C/350°F/gas mark 4 and bake for another 35–45 minutes, or until golden and the filling is bubbling up through the pastry. Cool for 3 hours before slicing.