Sift the flour, icing sugar and a pinch of sea salt into the bowl of a food processor.
Cube and add the butter, then blitz until the mixture resembles fine breadcrumbs.
Tip in 2 egg yolks along with 60ml of ice-cold water and gradually pulse the mixture until you have a rough dough.
Turn the dough out onto a clean surface and gently knead it for about 30 seconds, until smooth, adding a little extra flour if needed.
Divide it into two pieces, one slightly larger than the other, then roll them out into discs. Wrap both in clingfilm and chill in the fridge for 30 minutes.
Roll out the larger piece of pastry and press into a 23cm pie tin. Trim it so you have 2.5cm overhang and chill it in the fridge until required. Roll out the remaining disc and cut out eight long, even strips, each about 2.5cm wide.
For the filling, toast the almonds in a dry frying pan until golden. Halve and destone the plums, quartering any larger ones. Cube the butter, then place all the ingredients in a large bowl and stir well (depending on how sweet the fruit is, you may wish to add a little extra sugar).
To assemble, pile the fruit mixture into the pie base. Beat the remaining egg yolk with a splash of milk, then use it to brush the edges of the pastry.
Arrange and weave the pastry strips in a lattice pattern on top of the pie. Bring the overhanging pastry from the pie base up and over the ends of the pastry strips, pressing down to seal, then crimp it all the way around with your fingers and thumb.
Brush the pie all over with the egg wash and sprinkle with sugar. Pop it in the fridge to chill for 30 minutes.
Preheat the oven to 200ºC/gas 6 and place a baking tray inside to warm up.
Transfer the pie to the baking tray and bake it in the oven for 20 minutes, checking occasionally and rotating it if it’s colouring unevenly.
Reduce the oven to 180ºC/gas 4 and bake for a further 20 to 30 minutes, covering with a tent of tin foil if it’s browning too quickly.
Remove the pie from the oven, sprinkle with extra sugar and leave to set for at least 3 hours before serving.
Cut into slices and serve with ice cream or crème fraîche, if you like.