Herb and leaf quiche with mashed potato pastry

Method

1. To make the pastry, put the mashed potato and butter in a mixing bowl and add the flour, baking powder and a pinch of salt. Use the tips of your fingers to rub the butter and mashed potato into the flour until the mixture resembles breadcrumbs. Add the milk and mix thoroughly until you can form a smooth dough. Cover the bowl with a plate and leave to rest in the fridge for 20 minutes. Store in an airtight container away from direct sunlight for up to 3 months.
2. Meanwhile, fry the onion in the olive oil over a low-medium heat for 10-15 minutes, or until soft and lightly golden. Roughly chop the salad leaves and add them to the pan. Stir for a few minutes, or until wilted, then leave to cool.
3. Heat the oven to 180°C fan/gas 6. Once rested, roll the pastry out on a lightly floured surface to the thickness of a pound coin (3mm). Line a 23cm round tart tin with the pastry, then line that with baking paper and fill with baking beans or rice. Blind bake the pastry case for 20 minutes, then remove the beans (or rice) and baking paper and return the pastry case to the oven for a further 5 minutes, or until dry to the touch and a pale golden colour.
4. Reduce the oven temperature to 160°C fan/gas 4. In a large jug, whisk the eggs with the cream (and/or milk) and mustard. Season generously, then stir in 100g of the grated cheddar and most of the finely chopped herbs.
5. Evenly spread the onion and wilted salad leaves in the pastry case and pour over the custard mixture. Sprinkle the remaining cheddar over the top and bake for about 40 minutes, or until just set, with the slightest of trembles in the middle of the quiche. Leave to cool for 30 minutes before removing the quiche from its tin. Sprinkle with the remaining herbs and serve warm, at room temperature or chilled.