In a food processor, combine the onion, carrot and bacon and process until finely chopped.
In a saucepan over a medium heat, melt the butter and warm the oil. Add the onion and bacon mixture and cook and stir until soft, about 4 minutes. Sprinkle in the flour and give a good stir. Gradually add the stock, stirring continuously, then add the milk so that the sauce is rich and creamy. If the sauce is a little thick, add more liquid. Finally add in the cooked chicken and mix well. Transfer to a large pie dish.
Cut the pastry to match the circumference of the pie dish and place over the chicken, making sure to push the pastry into the edges (if you prefer to make this like a more traditional pie you can line the pie dish with the rolled out pastry before putting in the filling). Brush with an egg wash.
Bake in the preheated oven for 25 minutes or until golden brown and piping hot. Remove from the oven and enjoy!