GRAPE AND BROWN SUGAR GALETTE

• 100g plain flour • 150g wholewheat flour • ½ tsp fine sea salt • 60g light brown soft sugar • 175g unsalted butter, cubed and chilled • 1 large egg, separated • 2 tbsp cornflour • 3 tbsp blackcurrant jam • 400g black sable grapes • 20g flaked almonds • double cream, to serve (optional)