Ingredients

• 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)

Directions

• 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.