Heat a little oil in a medium saucepan, brown the chicken quarter, turning a couple of times. Remove chicken and leave to cool.
Bring leeks to the boil, remove and set aside.
Reheat the oil in which the chicken was cooked, add the onions, stirring for five minutes. Add a little leek water, a little bouillon powder, and so on until you have enough to cover the pie mixture.
Add the mushrooms, leeks and prawns, and stock cube, cook for five minutes. Break up the chicken from the bone and add to mixture. Season to taste (it will be salty already) and sprinkle with tarragon.
Upturn the pie dish, and cut around the pastry. Put the mixture into the dish, cover with pastry, add decorative pastry pieces.
Cook for 10-20 minutes on Gas mark 4 until pastry is nicely browned.
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