Method

• 1.
  For the pastry, combine flour, pepper and 1 tsp salt in a bowl. Toss through butter and lard. Using your fingers, rub into flour until it resembles coarse crumbs, leaving some larger pieces. Make a well in the centre. Combine sour cream and vinegar, then for flaky pastry add 2 tbs iced water, or for firm pastry add 2 tbs boiling water. Add to dry ingredients, then, using a small knife, cut together to form a dough. Transfer to a work surface lightly dusted with flour and knead into a ball. Enclose in plastic wrap and chill for at least 30 minutes to rest.
• 2.
  For the pie, place chicken, stock and half the oregano in a saucepan over medium heat and bring to a simmer. Reduce heat to low and simmer for 10 minutes. Strain through a fine sieve, reserving cooking liquid. Set the chicken aside to cool slightly, then use forks to roughly shred the meat.

• 3.
  Meanwhile, to make the sauce, place the mascarpone in a large saucepan over medium heat and cook, whisking, for 2-3 minutes or until melted. Add the flour and cook, whisking, for 2 minutes or until thickened and slightly golden. Gradually whisk 3 cups (750ml) reserved chicken stock into mascarpone mixture until smooth. Cook, stirring, over low heat for 3-4 minutes or until thickened. Add half the butter, whisking to combine, then add chicken, celeriac, lemon zest and remaining oregano. Season, then chill.
• 4.
  Preheat oven to 200°C.
• 5.
  Place leek on a microwave-safe plate and microwave on high for 10 minutes or until softened. Cool, then halve lengthways. Finely chop one of the halves and fold through pie filling, reserving remaining leek.
• 6.
  Grease a 20cm x 30cm ceramic or enamel dish. Roll out pastry to a 3mm-thick 25cm x 35cm rectangle and use to line dish, then fold overhanging pastry back on itself to make the sides doubly thick. Chill for 15 minutes to firm up.
• 7.
  Add the chicken mixture, then using your thumb and forefinger, pinch edges of pastry to form a fluted rim. Brush pastry with egg. Top with leeks, brushing with remaining butter. Cover the dish tightly with foil, then transfer to a baking tray.
• 8.
  Bake for 30 minutes, then remove foil and bake for a further 1 hour or until leeks and pastry are golden. Set aside to cool for 15 minutes, then serve warm.