Preheat the oven to 200°C (400°F).
In a large pan, melt the butter over medium heat and sauté the leeks for 5-7 minutes until soft.
Stir in the flour and cook for 2 minutes to form a roux, ensuring it does not brown.
Gradually whisk in the chicken stock, a little at a time, until a smooth, thick sauce forms.
Stir in the heavy cream, Dijon mustard, chicken cubes, and ham cubes.
Simmer the mixture for 5 minutes, then stir in the parsley and season with salt and pepper.
Pour the filling into a deep 20-22cm pie dish.
Place the puff pastry over the dish, trim the excess, and crimp the edges with a fork to seal.
Brush the top of the pastry with the beaten egg and cut a small cross in the center to allow steam to escape.
Bake for 25-30 minutes until the pastry is risen and deep golden brown.