FOR THE FILLING
4 unwaxed lemons
150g unsalted butter
250g caster sugar
2 large eggs
FOR THE PASTRY
75g soft unsalted butter
40g icing sugar
1 large egg, beaten
150g plain flour, plus extra for dusting
To make the filling, put the whole lemons in a pan and cover with water. Bring to a gentle boil, then simmer for 1-2 hrs, or until very soft, then drain and leave to cool. Cut the lemons in half and remove the seeds. Place the whole lemons in a food processor with the butter, sugar and eggs. Blend together, then set aside.
To make the pastry, cream the butter and icing sugar in a food processor (or use an electric mixer) until pale and fluffy. Add the egg, then
fold in the flour with a good pinch of salt. Mix to a dough, then roll out thinly on a lightly floured surface to a circle about 3mm thick. (It should be large enough to line a 25-26cm tart tin that is 2.5cm deep). Line a baking tray with baking parchment and carefully transfer the pastry to the tray. Put in the fridge to rest for 20 mins.
Line the tart tin with pastry, taking care not to stretch it, then trim the edge. Patch any cracks with the trimmings and reserve any remaining scraps. Chill again for 20 mins.
Heat oven to 180C/160C fan/gas 4. Line the pastry case with baking parchment and baking beans, then bake-blind for 25 mins, or until very pale golden. Remove the beans and paper, patch up any holes with the reserved pastry trimmings, then bake for a further 10 mins or until crisp.
Reduce oven to 160C/140C fan/gas 3. Fill the tart case with the lemon mixture and bake for 20-25 mins until lightly golden. There should still be a slight wobble in the centre of the filling. Leave to cool before slicing.