Rub together the flour and butter in a bowl, until it resembles fine breadcrumbs.
Mix in the egg with a knife, and then knead on a clean, lightly dusted work surface to form smooth dough. Wrap the pastry in cling film and chill for 30 minutes.
Preheat the oven to 190°C (fan 170°C, gas mark 5) and grease a 22cm loose bottomed flan tin (or you can make it in two 12cm loose bottom flan tins). Place the tins onto a baking tray.
Roll out the pastry until it's big enough to generously fit the tin. Ease the pastry into the corners of the tins leaving excess pastry falling over the sides - don't trim at this stage. Prick the base with a fork.
Line the pastry with greaseproof paper and fill with baking beans. Trim the excess pastry using a sharp knife. Bake the pastry case blind for about 15 minutes - remove the paper and beans and return to the oven for a further 5 minutes.
Mix the syrup, breadcrumbs, pecan, lemon juice and zest, eggs and double cream with a pinch of salt. Remove the tart from the oven and pour the filling into the pastry case.
Bake the tart for a further 40-45 minutes or until the filling has lightly set. Leave to cool and serve with crème fraiche or custard.