1) Combine the flour, granulated sugar and salt in the bowl of a food processor. Pulse 5 to 8 times to combine the dry ingredients. Add the cold butter, pulsing until the mixture resembles coarse crumbs. Add the water and apple cider vinegar to the flour-butter mixture. Pulse until moist clumps form; the mixture will not form a ball of dough.
Turn the dough onto a lightly floured surface and press into a disk. Roll out the dough on a lightly floured surface into a 13-inch (33-cm) round. Place into a 9 ½ -inch (23-cm) pie dish and decoratively crimp edges of the pie. Refrigerate for 30 minutes.
2) Preheat the oven to 350°F (177°C, or gas mark 4).
3) Toss the plums with brown sugar, balsamic vinegar and cornstarch. Pour the filling into the refrigerated pie crust.
4) In a medium bowl, stir together the granulated sugar, brown sugar, flour, cinnamon and salt. Blend in the butter with your fingers until the mixture forms small clumps. Stir in the almonds. Crumble the streusel evenly over the plum filling.
5) Place the pie on a parchment-lined baking sheet. Bake for 50 to 55 minutes until the crust and top of pie are golden and the filling is bubbling.