Put the flour, butter, icing sugar, ground almonds and zest in a food processor and whizz to crumbs. Mix the yolk with 1 tbsp cold water, then add to the processor and pulse until the dough comes together.
Split the pastry in half, wrap in cling film, then chill for 30 mins.
Heat oven to 190C/170C fan/gas 5. Line a 20cm pie dish with half the pastry, add a sheet of greaseproof paper and baking beans and bake blind for 30 mins. Remove the paper and beans. Mix the berries and cornflour with the extra 2 tbsp icing sugar and a squeeze of orange juice, then pile into the pie base.
Roll out the remaining pastry until big enough to cover the pie with a slight overlap. Whisk the egg white with a fork, then brush a little around the pastry rim. Carefully lift on the pastry lid, trim the edges and press with fork prongs to seal. Use a 2cm cutter, the large end of a piping nozzle or a bottletop to stamp out a few pastry polka dots – discarding the pastry circles you stamp out. Brush all over with more egg white, then sprinkle generously with granulated sugar and bake for 30 mins until golden and the syrupy fruit has started to bubble out of the polka dots.
Allow the pie to cool for a few mins before serving hot, or warm or at room temperature, with a dollop of clotted cream, ice cream or drizzle of single cream, if you like.