First of all prepare the pastry. Mix the flour and sugar together with a fork. Make a well in the centre and add the butter. Rub the butter into the flour until the mixture resembles fine breadcrumbs, then add the lemon zest. Stir in the egg and vanilla until the dough becomes more compact and does not crumble. Make a ball and cover it with cling film. Leave it to stand in the fridge for half an hour.
Preheat the oven to 180 C / Gas 4. Line a tart tin with baking paper.
Roll out 3/4 of the dough to a height of 4-5mm and line the tart tin. Spread the clementine marmalade onto the pastry. Roll out the remaining pastry and cut into 1.5-2cm strips. Use to decorate the top of the tart in a lattice pattern.
Bake for 35 minutes in the preheated oven. Cool the tart in the tin, then cut into wedges and serve either warm or cold.
Goes nicely with a spoonful of Greek yoghurt, creme fraiche or ice cream.