- 100g/4oz plain flour, plus extra for dusting
- salt and freshly ground black pepper
- 1 tsp English mustard powder
- 75g/3oz chilled unsalted butter, diced
- 100g/4oz leftover mashed potato
- ½-1 tbsp iced water (optional)
- 2 tbsp olive oil
- 400g/14oz leeks, trimmed and thinly sliced (225g/8oz trimmed weight)
- 100g/4oz chestnut mushrooms, brushed to remove any grit, sliced
- salt and freshly ground black pepper
- 75g/3oz mascarpone
- 1 tbsp wholegrain or Dijon mustard
- 50g/2oz Cornish blue, Stilton or Gorgonzola (or a similar vegetarian blue cheese), diced
- pinch crushed dried chillies (optional)
- lightly dressed fresh green salad, to serve (optional)
- Preheat the oven to 200C/400F/Gas 6.
- For the potato pastry, sift the flour into a bowl and season well with salt and freshly ground black pepper. Stir in the mustard powder.
- Add the diced butter and, using your fingertips, rub the butter into the flour mixture until it resembles fine breadcrumbs.
- Add the leftover mashed potato and pull the mixture together using your hands to form a soft dough. If necessary, add a little iced water to the mixture to loosen it, but take care not to make the dough too sticky.
- Turn out the dough onto a lightly floured work surface and knead lightly for 2-3 minutes. Roll the dough into a ball, wrap in cling film, and chill in the fridge for at least 10 minutes (or for up to 24 hours, if time allows).
- Meanwhile, for the filling, heat the oil in a frying pan over a medium heat. Add the leeks and fry gently for 5-7 minutes, or until softened but not browned.
- Add the mushrooms, season to taste with salt and freshly ground black pepper, and continue to fry with the leeks for 4-5 minutes, stirring occasionally, until just softened. Set aside to cool.
- When the leek-and-mushroom mixture has cooled, stir in the mascarpone and mustard until well combined. Season, to taste, with salt and freshly ground black pepper.
- When the pastry has rested, roll it out onto a lightly floured work surface to a thickness of a pound coin.
- Line a baking tray with baking paper, then transfer the rolled-out pastry to the lined baking tray. Pinch the edges of the pastry to make a rough tart case with a rim.
- Pour the filling mixture into the tart case, then sprinkle over the cheese and the crushed chillies (if using).
- Bake the tart in the oven for 25-30 minutes, or until the rim of the tart case is golden-brown and the filling has set. To serve, cut the tart into slices and place one slice onto each plate. Serve the green salad alongside.