Preheat oven to 190C.
Heat oil & butter in a large frypan over medium heat. Add onion, leeks, carrots, salt and pepper, then saute for 5 mins or until softened. Add the mushrooms and wine and cook for few minutes more. Add the chicken and peas, remove from heat and set aside.
Now for the sauce: melt the butter in a saucepan over medium heat. Add the flour and cook for a minute, stirring all the time. Slowly add the hot stock and cream, whisking continuously, and simmer until sauce thickens slightly. Whisk in mustard & nutmeg and stir through the herbs. Season with salt and pepper.
Stir the sauce through the chicken and vegetables and pour into a pie dish.
Roll out the pastry to approx 1 cm thick and large enough to cover the dish. Roll up on your rolling pin and then unroll over the filling. Press down gently around the edges and trim off any excess pastry. Cut an ‘x’ in the centre to allow the steam to escape. Cut out some decorations from the pastry scraps and position them on top of the pie. Brush with beaten egg.
Place in the preheated oven and bake for 35-40 minutes or until pastry is puffed and golden.