Preheat a baking sheet in the oven to 200°C.
Blend flour and butter in a food processor until the mixture resembles breadcrumbs.
Add egg, sugar, and water, then blend into a dough.
Roll out the dough on a floured surface to a 1cm thickness.
Cut the pastry into a 25cm circle.
Spread mincemeat over pastry and bake on the hot sheet for ten minutes.
Melt sugar in a frying pan over gentle heat to create a smooth caramel.
Mix roasted chestnuts into the deep amber caramel.
Add lemon juice and pour onto a greased baking sheet.
Break the cooled caramel mixture into pieces.
Serve the tart with ice cream and caramelised chestnuts.