Make the pastry

1. Break the egg into a 2 c measuring cup and whisk. Add the vinegar and enough cold water to make 135 ml or 4 1/2 oz of liquid. Whisk until well combined. Place in the fridge.
2. In the bowl of a food processor, pulse the flour, sugar and salt until combined. Add the butter and pulse until lumps of butter are pea-sized. With the machine running, quickly add the egg mixture through the chute. Let run for a few seconds until the machine makes a heavy rumbling noise. Turn off immediately; do not overmix. Using a rubber spatula, scrape the dough into a large, cold bowl. Carefully press into a large ball, closing cracks and forcing out air. Cut the ball in half and wrap each half in plastic wrap. Refrigerate for at least an hour.

Prepare the fruit fillings

1. In a medium pan, combine the drained cherries and 2/3 c of the water in which they were soaking, along with the sugar and tapioca. Heat, stirring constantly until the mixture thickens (5 minutes). Place the pan in the sink partially filled with iced water and allow the cherry mixture to cool.
2. In a large bowl, squeeze the juice of half a lemon. Add the squeezed lemon half and enough water to cover thin slices of two Granny Smith apples. Core the apples and slice very thinly and evenly, depositing the slices in the lemon water. Set aside. Pit and thinly slice four large black plums, putting the slices onto a plate. Set aside.

Assemble the pie

1. On a lightly floured board, roll out one of the balls of pastry to 1/8″ thickness. Fit it into an 11″ tart tin with a removable bottom. Roughly trim the pastry to 1″ beyond the rim of the tart shell. Tuck the excess between the pastry and the tart tin to make an even outer crust. Place on a rimmed baking sheet lined with parchment paper and transfer to the fridge. (The parchment paper will make clean-up a lot easier).
2. Roll out the second ball of pastry and cut out approximately 50 leaves, placing them on a baking sheet lined with parchment paper. Refrigerate.
3. Fill the pastry-lined tart shell with the cooled cherry pie filling and smooth the top. Arrange alternate slices of apple and plum around the perimeter of the shell. Place leaves around the outside edge of the pie and in concentric circles in the centre, starting at the edge of the apple and plum slices and working your way in. Place the pie in the freezer for 15 minutes.
4. Preheat the oven to 400° F.
5. Beat an egg with a few drops of water. Apply egg wash to the leaves using a small artist’s paintbrush. Bake the pie until the filling is bubbling and the leaves are golden brown. If browning too quickly, loosely cover the pastry with foil.
6. Let the pie cool completely before slicing and serving.