Preheat the oven to 200C/400F/Gas 6.
Heat the butter in a pan and gently fry the onion until softened but not coloured.
Add the bacon and fry until golden-brown.
Add the chicken pieces and cook for 4-5 minutes, stirring frequently, until lightly cooked on all sides.
Add the peas and double cream, season with salt and freshly ground black pepper and simmer for 2-3 minutes. Pour the mixture into an ovenproof dish.
Roll the pastry out on a lightly floured surface and cut the pastry to fit over the top of the dish. Cover the chicken mixture with the pastry and cut a steam hole in the centre.
Transfer to the oven and bake for 15 minutes, or until the pastry is golden-brown and the chicken is completely cooked through.
To serve, place servings of the pie onto plates.
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