Preheat a baking sheet in the oven to 200C/400F/Gas 6.
For the pastry, place the flour and butter into the bowl of a food processor and blend until the mixture resembles breadcrumbs.
Add the egg, sugar and water to the bowl and blend until the ingredients have combined to form a dough. Remove the pastry from the food processor and bind it together with floured hands.
Place the pastry onto a floured surface and roll it out to a thickness of 1cm/½in.
Cut the pastry into a 25cm/10in circle.
For the filling, spread the mincemeat over the pastry and transfer the tart to the hot baking sheet. Bake for ten minutes.
For the caramelised chestnuts, heat the sugar in a frying pan at a gentle heat, until the sugar has melted to create a smooth caramel.
When the caramel has turned a deep amber, add the chestnuts to the pan and mix well.
Add the lemon juice and shake the pan to mix. Pour the mixture onto a greased non-stick baking sheet.
Once the mixture has cooled and set, break it into pieces.
To serve, place the tart onto a clean plate and dust it with icing sugar. Place four scoops of vanilla ice cream on top of the tart and sprinkle the broken caramelised chestnuts on top of the ice cream.