Begin by making the pastry. Place the butter, flour and salt into a food processor and blend until it has an even, sandy texture, then add the egg and pulse the mixture until it comes together into a dough. Tip out onto a work surface, gather into a ball and then pat down into a flat disc. Cover in clingfilm and chill for 1 hour.

In the meantime, place a large nonstick frying pan over a medium heat and add the butter. Once melted, add the leeks and sweat down for 10 minutes – they should be soft but still bright green.

In a bowl, mix together the cream, egg, mustard, chives and two-thirds of the grated cheese. Take the leeks off the heat and stir in the mixture, then leave to cool completely.

Grease and flour a 22cm fluted tart tin with a removable base. Roll the dough out in between two sheets of greaseproof paper then transfer to the tart case, leaving a few centimetres of overhang all the way around. Prick the base with a fork and place back in the fridge for 30 minutes. These resting steps are important to stop the pastry shrinking during cooking.

Preheat an oven to 170°C/gas mark 3. Place a sheet of foil in the pastry-lined tart case and mould it to the same shape. Fill with baking beans or dry rice and blind-bake for 10 minutes. Remove the beans and foil and cook for 5 more minutes, then remove from the oven. Once cooled slightly, use a small serrated knife to carefully carve off the pastry overhang.

Spoon in the cooled leek mixture and spread out evenly. Sprinkle over the leftover cheese then bake for 25-30 minutes. Best served cold or at room temperature.