To make the pastry, tip the flour, butter and sugar into a food processor with a pinch of salt. Whizz until the mixture resembles breadcrumbs. Add the egg yolk and 1 tsp of cold water and pulse until the dough comes together. Flatten into a disc, cover with clingfilm and chill for no more than 1 hour. n Roll out the pastry on a lightly floured surface to about 3mm thickness. Line a 20cm fluted tart tin with a depth of 31/2 cm. Prick the base with a fork and chill for 20 minutes. Heat the oven to 180C/fan 160C/gas 4.
Line the pastry case with baking parchment and fill with baking beans. Cook for about 20 minutes until the pastry is a pale golden colour. Take out the beans, brush the inside of the pastry case with a little egg white and cook for a further 2 minutes. Cool slightly.
Spread the jam in an even layer over the base of the pastry case. Cream together the butter and caster sugar. Gradually add the beaten eggs and egg yolk. Fold in the ground almonds and lemon zest. Carefully spoon the mixture over the jam and spread level. Bake for 20 minutes. Scatter with the flaked almonds and continue to cook for a further 15-20 minutes until golden and set.
Cool to room temperature, dust with icing sugar and serve with pouring cream or custard.