For the pastry, place the flour and butter into a food processor and pulse until the mixture resembles breadcrumbs.
Add the egg yolk, sugar and about two tablespoons of cold water and pulse until a firm dough is formed.
Remove from the processor and place onto a lightly floured surface. Knead lightly, then wrap in cling film and transfer to the fridge to chill for 30 minutes.
Preheat the oven to 180C/355F/Gas 4.
Remove and unwrap the pastry and roll out the pastry on a lightly floured surface. Line a 25cm/10in shallow, fluted loose-based flan tin. Press the pastry into the sides and prick the base all over with a fork. (Save any spare pastry for decoration.)
For the filling, gently heat the golden syrup in a pan until melted down and runny. Remove from the heat and mix with the breadcrumbs and lemon zest.
Add the eggs and stir well.
Pour the treacle mixture into the pastry case. Use any spare pastry to decorate the top with lattice strips. Transfer to the oven to bake for 45-50 minutes, or until the filling is lightly set and golden. Allow to cool slightly before removing from the flan ring and serving.