Preheat the oven to 200°C (180°C Fan).
Melt the butter in a saucepan and gently cook the sliced leeks until softened.
Stir in the flour and cook for 1 minute to make a roux.
Gradually whisk in the chicken stock followed by the milk until smooth and thickened.
Stir in the double cream, Dijon mustard, parsley, salt, and black pepper.
Fold in the diced chicken and ham, then remove from the heat and allow the filling to cool slightly.
Spoon the filling into a greased pie dish.
Cover with the puff pastry, trim the edges, seal well, and brush with beaten egg.
Cut a small vent in the centre of the pastry.
Bake for 30–35 minutes until the pastry is puffed, crisp, and deep golden brown.
Rest for 10 minutes before slicing and serving.