1
Prepare the Leek. Trim the leek, halve it lengthways and slice it about 1 cm thick. Wash the slices in a colander, pour over a kettle of boiling water, drain and leave to dry.
2
Fry the Chicken. Heat the oil in a large frying pan over a medium high heat. Fry the chicken for 5 to 6 minutes, until golden and cooked through, then lift it out.
3
Soften the Leek. Turn the heat down, add the butter and the leek and cook gently for 5 minutes, until soft but not coloured.
4
Make the Béchamel. Stir in the flour and cook for 1 to 2 minutes. Add the milk a splash at a time, stirring all the time, then simmer for 3 to 4 minutes until thick.
5
Finish and Cool the Filling. Stir in the Parmesan and a little grated nutmeg, put the chicken back and season to taste. Leave the filling to cool completely. It thickens as it cools.
6
Fill and Top. Heat the oven to 200°C (180°C fan). Spoon the cold filling into one large pie dish or 8 small ones. Lay the pastry over, press it to the rim and trim. Brush with the egg, grind over black pepper and make one small poke in the middle.
7
Bake. Bake a large pie for 30 to 35 minutes, or small pies for 20 to 25 minutes, until the pastry is puffed and deep golden and the filling is bubbling at the edges.
8
Rest and Serve. Leave to rest for 5 minutes, then serve with mashed potatoes.