Preheat the oven to 150°C/300°F/ Gas mark 2. Put in a large heavy saucepan with the chicken wings.
Add the onion, leek and parsnip, and bring to the boil. While the water is heating, add the whole peppercorns.
Once the stock has boiled, transfer to an ovenproof dish and cook in the oven for 2 hours. Skim off any scum from the surface using a slotted spoon, and strain the stock you have created. Discard everything except for the chicken.
When cool enough to handle, take out the carcass and shred the chicken before returning it to the stock. Serve hot.