Put the flour and cinnamon in a food processor, then add the butter. Process to make fine crumbs. Reserve 2 tbsp of the sugar, then add the remainder and briefly mix.
Add the egg yolk and 1 tbsp water, then pulse to make a firm dough. Turn out onto a lightly floured surface and briefly knead. Wrap in cling film and chill for 30 mins.
Toss apricots in the reserved sugar.
Roll out pastry on a sheet of baking parchment to a roughly 30cm round. Slide the paper and pastry onto a large baking sheet.
Sprinkle the central 20cm with the polenta, then cover with apricot halves, cut sides up. Flop edges of the pastry over the apricots, leaving the centre uncovered. Lightly beat egg white and brush over the pastry.
Sprinkle with remaining sugar, then bake for 30-35 mins until the pastry is crisp and the apricots tender.
