Heat the oil in a flameproof casserole over a medium-high heat and fry the pancetta until browned and crisp, about 5-7 mins. Remove to a bowl using a slotted spoon. Tip the chicken into the pan, season well and fry for 5-7 mins until browned all over. Remove to the bowl with the pancetta. Fry the mushrooms in the pan for 7-8 mins until browned and crisp, then stir in the butter, the white parts of the spring onions and the flour. Cook for a few minutes more until the spring onions have softened slightly, then slowly pour in the stock, stirring well after each addition.
Bring to a simmer and mix in the mustard, bay, nutmeg, mustard powder and thyme, then return the chicken and pancetta to the pan. Simmer for 20-25 mins until the mixture has reduced slightly and thickened, then stir in the peas, double cream and green parts of the spring onions. Season well and tip into a 18 x 28cm pie dish, then leave to cool for 10-15 mins.
Heat the oven to 200C/180C fan/ gas 6. Lightly dust a clean work surface with flour, then roll the pastry out to roughly the thickness of a £1 coin. Brush the edges of the pie dish with some of the beaten egg, lay the pastry on top and trim to fit. (You can use the offcuts to decorate the pie, if you like.) Cut a steam hole in the middle. Once completely cool, the pie can be covered and frozen for up to three months. Defrost in the fridge overnight before baking as directed. Brush the pastry all over with the remaining beaten egg and bake for 30-40 mins until piping hot and deeply golden. Serve with veg and new potatoes, if you like.
